|12-02-2004 06:51 AM|
I recognize a few names on this post from other forums. Hi guys!
The talk about sand blasting is interesting...sounds like it might make a great stretching tool.....since I never use sand blasting, I haven't given much thought to its effects..stretching and work-hardening. A shrinking disc will work on large areas, but on a low crown panel, you may need to prop the metal out to shrink it without it popping in on you. As far as which side of the metal is stretched or shrunk, I don't think of it that way. It does not matter which side you apply shrink, or which side you hit with a hammer other than the convenience of access, or to bump the metal in a certain direction, which has nothing to do with stretching or shrinking. You can figure out whether shrinking or stretching is required by taking a template of the curve up and down and another front to back from the other side of the car. You need to read the panel in both directions to see if it is low or high. It is easy to get a false read if you don't check this in both directions as you work. Sometimes you might have to smooth with a hammer and dolly and shrinking disc just to read the panel, and then proceed to stretch up low spots, or shrink down highs, or both.
I've used a sauce pan lid, the bottom of a small stainless pitcher, a clogged up scotch brite pad on a sanding disc backup pad to shrink...they all work, but a shrinking disc takes the cake. If you see steam when you quench, you are shrinking. No need to change the color of the metal. You can shrink down huge bulges, or tiny little areas. Lately I've been playing with a low crowned disc (think very shallow hubcap) which seems to work even better than a flat one. It works in shallow reverses, and on flat panels where a flat disc is problematic, and it also works on regular crowned panels.
Oil cans are relatively easy to fix once you realize you can shrink and stretch at will without worrying, because you can always correct an over-stretch, or over-shrink.
By the way, the how to make a shrinking disc article was by Paul on the metalmeet site.
|11-30-2004 08:04 AM|
Great stuff Randy, thanks for posting it. I saved that one!
Like I have discussed with you, every time I read something you write something you have been saying finally hits home in my head. You have been talking about "slappers", I know some of the uses, I have a couple. But Wrays article really hit me as to why and how much more I should be using them. You could have wrote it again and it would have hit home just the same of coarse being Wray wrote it doesn't make the difference, I only mean that it FINALLY hit home with me.
Thanks again, Brian
|11-30-2004 07:41 AM|
Thanks Randy, "knowledge is power"!!
I previously found an article by Wray with pictures on how to make my own disk, it's great. A local restaurant supply has commercial stainless cookware lids that are the right thickness, diameter and flat. I just had to cut off the handle that was spot welded on, cut off the lip and drill the hole for my grinder. I heated and offset the hole about a 1/4" for the grinder nut.
|11-29-2004 06:55 PM|
If you insist on trying to fix the problem and you do not have a shrinking disc, I would suggest getting one from Wray Schelin @ WES Parts. (508) 347-7749.
The following is a write up he did on the Jaguar forum a few years ago.
Although you are working on more than just a dent, the process is the same. the low area must be brought up equal to or above the surface level befor using the disc.
Tools for Removing Dents
by Wray Schelin
As promised to the XK self-restorers, who are about to get to the paint stage.
How smooth are your stripped body panels? That is the question you need to know before you advance to the primer stage. If they're not smooth you can fill them with bondo and heavy primer- but if you do so you run a high risk of a early paint failure and all your work will be in vain- or you can smooth them by accurately working the panel back to a smooth condition. If you rub your hand over the metal and feel low spots or high spots, you can be sure that they will show . Before you get your spray gun out you should be able to rub a panel in any direction and feel nothing but smoothness.
If you choose smoothing instead of filling, I'll share with you the technique that I use and some of the tools needed.
First the tools:
All body tools should have smooth working surfaces; hammers, slappers, and dollys are like printing presses, they will transfer the imperfections on their working faces to the metal over an over. Its best to take the time and smooth your tools first , because any imperfection transferred to the panel can make itself known later in the topcoats,as a paint shrinkage depression.
The higher quality body tools are made of heat treatable steel. You should have at least one hard hammer and one hard dolly- for hammering welds. The reason for this is the welds are harder than the surrounding metal and if you flatten them with a non hard hammer you will mark the hammer and then you will have to keep smoothing it. The working faces of hammers and slappers should have a very slight crown to them and the edges should be radiused. With the edges smooth and the center crowned slightly, you will not mark the panel if you inadvertantly strike with the edge of the tool.
The slapper is the most important smoothing tool. You can make a slapper out of a old rear leaf spring. Car springs are harder than welds, so they will not mark up. My slapper has a working face of 2.250" by 5". The thickness is .250" and the unbent starting length is 14". You can also get a single spring leaf new from your local truck spring rebuilder. You can cut it to shape with a cuttoff wheel or a torch, but cool it quick so you don't anneal it. I bent mine with an offset of 1.625" . The offset allows you to affix a wood handle and provides the clearance for your fingers. Heat with a torch to bend the offset and then narrow the handle end to a width of 1.375". The narrowed part is 5" long.
This slapper will smooth all body panels except for concave areas. For concave areas you will need to make a special slapper or use hammers. Once you start to use a slapper you will retire your hammer.
You will also need a few dollys. I found most of my dollies at flea markets for a few dollars each. Three or four dollies will be more than adequite to deal with all the different shapes and contours that you encounter . Each dolly usually has several different contours and crowns. As long as you have a straight edge , a low crown, medium crown, and a high crown you will be able to smooth any panel. Don't hesitate to alter the dolly to fit a need ,grind them with a body grinder, to rough shape then use a DA sander- with finer and finer sand papers- in rotary mode to achieve a fine polished finish.
Next you will need a body file and holder. The holders have a turn-buckle on them to allow you to flex the file to a concave, flat, or convex shape. The file that I use I was able to order from my local welding supplier. They are a dealer for a German company called Pferd. I got a Pferd catalog, an found that they offer a 12 tooth per inch body file , which is considered a fine cut body file. Most of the files that I had seen previously were 8 or 9 teeth per inch which are coarse body files. I like the fine file because I can use it on aluminum , steel, or body solder. When I use it on aluminum I load the teeth with a candle wax, this allows you to skate over the aluminum without digging in and making gouges. This type of file is 14 inches long and has cutting surfaces on each of its sides. On one side I grind the edges smooth in effect killing them so they do not dig in as you skate the file sideways. Pferd also sells the holders and a mutitude of different style files and abrasives, all are of the highest quality available anywhere. Pferd has distribution centers all over the world. In the USA they can be reached at
30 Jytek Dr.
Leominster, MA 01453
Phone 508 840-6420
Pferd Australia (Pty) Ltd.
3189 8 Capella Cresent
You can also try your local welding supply house and they might have a catalog.
Another item I use is a large magic marker or felt marker. The ones that I use are called magnums and they mark a swath about 1/2" wide with a tenacious ink that dries very quickly. I like red ink the best.
A heavy duty 9" body grinder is the most expensive item needed. You can use a lighter duty 7" grinder but it won't work as well as the 9" in all cases.
Also a 9" 120 grit grinding disc. Grinding discs when they are new are very sharp when you run your finger over them , after grinding a heavy peice of steel for a few minutes you will dull the disc. This is how I prepare my discs, purposely dulling them to make them suitable for use.
Lastly you will need the Amazing Shrinking Disc. I mentioned this tool before in another post, it is a 9" disc of .050" stainless steel. This tool is most effective when used with the heavy duty 9" body grinder. I can't say enough about how good this simple tool works.
In my next post I will explain the process that I use in conjunction with the earlier mentioned tools. For many years I haphazardly removed dents with a method which always left the panel in a improved state, but not perfect. I frankly didn't believe you could restore the damaged metal to a state were no filler other than primer would be neccessary. Like most things once you master them they are quite simple, all you need is the determination and the correct method.
Wray E. Schelin
As Paul Harvey says "And now, the REST of the story!"
by Wray Schelin
In the last post I described the tools neccessary to completely remove
dents, waves, and dings in your body panels. In this post I will share with
you how I use the tools to achieve a panel smoothness that will require
very little or no bondo filler. It is best to keep your bondo use down for
two reasons. One,bondo use is not craftsmanship, its just a cheap
substitute. If your trying to achieve a high standard restoration- in my
opinion it is best to have the craftsmanship on more than just what you
see. The value of these cars is more than just the dollar amount. The
second reason is, if you keep your substrates (bondo fillers and primers)
to a very minimum and apply just enough topcoats you will have an ideal
thickness of paint coatings. With an ideal thickness your paint system will
be able to expand and contract with the steel and aluminum surfaces of your
cars body as it heats and cools. This correct thickness insures you against
an early paint failure. Coatings can fail for many other reasons, but too
much paint and filler I believe is the most common culprit.
The most surprizing thing about high quality metal finishing, is that is
not that difficult to do; but it does take patience, good eyesight, a fine
sense of touch, and the tools that I previously mentioned.
First what's fixable and what's not. If you have, say, a 120 front fender
that was severely damaged in a accident many years ago on its leading
surfaces, an was quickly repaired by sewing up tears with brazing rod,
crudely hammered out, ground very thin, and then filled with bondo, forget
it; in that case you are probably better served by replacing that heavily
damaged section. Another impossibilty is an area that has been incorrectly
torch shrunk; what had started out as an earnest limited attempt,
inadvertantly expanded to large area, leaving heat damage with heavy
Fixable dents and damage, listed in a descending order of severity:
Bodged past repairs that are still fixable, because the metal has not been
ground too thin,
Collision damage with stretching and tearing,
Sandblasting with excessive pressure causing a wave effect,
Smoothing out the seam of a butt welded patch panel,
Small dents with little or no stretching,
I 'll share with you how I remove a small dent . An easy example will work
best, so lets say, its the rear fender of a XK120. The dent is in the
middle of the rear section of the fender, and its is about the size of your
fist, sunken in about 3/4" in the center. All paint and undercoating should
be removed first. I would first select a dolly that has a crown that is
close to the fender; in this case that would be a medium crown. Using a
glove to protect my fingers I would palm the dolly and lightly tap it
against the bump on the inside of the fender; carefully watching the
progress of the rising depression. I would use this process until I got the
dent up to within 1/8" of the surface. This will happen within minutes-
this is called roughing out the dent. Next I would hold the dolly tightly
against the center of the damage , on the backside, while I use the slapper
on the front, tapping the circumference of the dent. This is a dolly off
action, the slapper and dolly are not clashing with each other, they are
beside one another. I would keep tapping away with the slapper, moving the
dolly tightly with some force, against the lowest area of the dent. Slowly
the dent will rise to very close to the surface level. The slapper does
this operation very effectivly because it has such a large surface area,
compared to a hammer. With a hammer you're hitting a smaller area and you
might dent the area you're hammering against because it will yield easier
than the center of the dent.
Roughing and slapping the dent has reduced the dent by about 90% and
progress was swift. The next stage of metal finishing requires the bag of
tricks and the tools. The problem that you encounter at this final stage
is, you have trouble seeing what you're doing because your actions have to
be small. When you were roughing you could easily see the metal move closer
to the surface; but now you might only have to move the metal forty
thousands of an inch or less to reach the true surface. At this stage a
common practice is to use a pick hammer. In my opinion a pick hammer has
many drawbacks: one- you need room to be able to swing it; and generally
the hammer itself might be 6" or more across the head. Two- it is very easy
to over hit with a pick hammer and cause irrepairable damage. Three- more
likely than not you will not be able to strike the low spot, instead you
will hit the high spot worsening the problem. The safest bet is to retire
your pick hammer. This verdict also applies to the bulls-eye gimmick tools
which use a C shaped frame to guide you to the elusive low spot. If you go
down the bulls-eye road you will find your garage populated with many
expensive sizes and versions abslolutely needed to remove all those pesky
dents and dings. You will allways be one bulls-eye tool short.
What I do at this stage is coat the damaged area entirely with the 1/2"
wide red magic marker ( thats a US trade name for those who might not be
familar with them- there is no magic, its just a felt ink marker) Next I
draw the fine body file over the area, just lightly skimming the surface,
this will quickly reveal the high and low spots. The object now is to raise
the low spots. You can do this by placing a dolly with a high crown surface
tightly against the low spot. You will only be guessing at this point
unless you have x-ray vision . You find out where you really are with the
dolly by lightly slapping the surface, with the the slapper a few times,
trying deliberately to strike the dollies crown. If you are successfull -
and you probably will be, because of the slappers large working surface-
you will hear the ring of the contact of the metals. Slide the slapper to
the side, but leave the dolly where it is. You should be able to see a 1/8"
diameter ( a 1/8" inch affected area will raise quickly with little force ,
the size of the mark made when you slap it determines the speed of the
metal rise. 1/8" is fast 1/2" is slow) clear spot, or slightly less inked,
in a region of the small low spot that you were raising. If you goofed and
hit a high area instead you should be able to see a difference there too.
Whether you were in the right area or not is not important, what is most
important, is establishing where you are and being able to adjust . Watch
the trail marks left in the inked surface and you can steer the dolly, on
the backside, easily to where it is needed. Slap lightly, slowly raising
the low spot. After a few minutes, refile the area and your progress will
You might have to wash off the marker ink ; re-ink, and refile several
times. Each working of the area will reduce the size of the low spots.
Remember that the filing is meant just to scrape off the ink and not to
reduce the thickness of the metal. The force and stroke of the slapper will
be less as you progress. When you have reduced the low areas to less than
1/2" in diameter, and when you rub your hand over the area you still
slightly feel them, you are ready to use the shrinking disc.
The condition of the metal at this point is stressed and springy as a
result of all the trauma inflicted on it. The original damage has been
raised but in the process the metal has been stretched a little. If you
applied bondo at this stage some of the bondo would surround the damaged
area, feathering in the new surface height.
With the marker ink still on, and a wet rag handy, crank up the body
grinder with the shrinking disc and rub the area. You vary the pressure
according to how much you need to shrink. On the first pass I usually apply
light pressure. The metal will quickly begin to rise and expand from the
heat build-up. Remove the disc and wipe the area with a wet rag. With that
operation you have started to shrink, stress relieve and further fine tune
the outline of the low spots. You can now re-mark with ink, file and
further tap out the low spots with the slapper and dolly. Some dents might
require several cycles, but as you hone your technique you should be able
to remove most dents in fewer cycles . At this stage an obvious high spot
might have developed. You can easily remove it by rubbing the shrinking
disc over it; it will heat to a blue condition in seconds, and then cool
with the wet rag. After I'm satisfied that I can no longer effectivly
raise any remaining tiny low spots (depressions only a few thousands of an
inch deep). I then install the very dull 120 grit 9" grinding disc and
proceed to work the area with it. The grinding disc will level the area
leaving a almost polished surface, it will heat the area quickly also, so
cool it with a rag after you done grinding. If you have done everything
correctly you should have a very smoooth surface , that is stress free and
in no need of bondo.
If you practice these tecniques on some old , damaged, and unimportant
sheetmetal parts you will quickly hone your skill.
On some areas of the XK Jaguars it is almost impossible to get a
conventional dolly into the area; in those cases you have to be resourceful
and fashion something that will snake into the damaged area, it will be
effective as long as it resists the blows of the slapper causing the metal
When working aluminum, I use all of the same techniques except, I do not
use the shrinking disc. Aluminum is much softer and requires less force to
Wray E. Schelin
One other point I just notice is in using the shrinking disc on aluminum. We now have discovered the disc doesn't need the ruffled edges, so we are making our own with a 1/4" flange. You can use common bar soap on the disc and a lttle rubbed across the panel and the disc works great for shrinking aluminum.
|11-29-2004 07:48 AM|
sandblasting - not my first rodeo!
Wow, it's amazing how juiced up people can get about a little thing like pushing sand particles at 100 mph!!! Gotta love this forum!! Thanks guys for being as passionate as I am to do things right!!
The only things I've been getting blasted are the heavy items like frame, brake shields and brackets. I specifically asked the guy to do the door edges and to just remove the worst scale at low pressure. The other items had come out great.
Thanks Randy for answering my real question about how to approach the problem. I'll try the disk, should be fine with that because there's really not much stretching. Just wanted to know if there were any tricks to finding the areas to concentrate on. If for some reason that doesn't work, I'll pull the doors off of the parts truck and go with those.
Makes me laugh at myself though, I'd rather spend the time to learn how to master the strectched metal (now that it's jacked up) than to just go pull those other doors!!
|11-26-2004 10:08 AM|
It comes down to common sense guys, a little experience is what it takes also. I sand blast a lot of parts, but never a large panel, hoods, decks, doors etc.. I blast firewalls, door jambs.inner structures, rockers. bumpers. floor boards, inner fenders, cowling ,tailgates, windshield posts, frames and suspensions etc.
Last year when I was doing my elcamino, I was using a small spot blaster to clean the windshield pinch weld area, the hood had a couple of scale rust spots the size of a quarter and I hit them with the little blaster, they must have raised a 1/4 inch.
It took me a week to fix that dang thing.
Like the rest, I have seen many parts and whole cars ruined with a sand blaster, but I blast every thing I can,t sand easily. I shoot at an angle, never 90* and use a silica sand. I also try to keep the nozzle as far away as I can.
I have blasted lots of early rod body shells with no problems.
You just need to know what you can blast and what you can't.
It's like any other tool, just use it wisely.
|11-26-2004 09:25 AM|
I think what XXXXXX writes in that message, and what I have written above mean about the same thing. I put things into laymans terms and he took the technical route. THe only difference I can see would be the stretching of only the surface. If this were the case, then simply sanding the hardened, compressed surface would relieve the stresses and the rest would pop right back up into place. At least that how I understand it would be if that theory were correct. Not being a scientific kinda guy, I couldn't care less how the molecules of metal are stacked. I can't see them, so to me, they are a non-issue. As long as you can shape a piece of metal to look how you want it and it not be stressed and under tension, then you've done your job well. This is a whole other story for another day.
The question here was how to fix the stretched metal from sandblasting. It can be done effectively with a shrinking disc, but be prepared to spend several hours trying to get it right. My advice: get another set of doors.
And remember my motto "Metal is stupid. You control the metal, the metal never controls you."
|11-26-2004 09:14 AM|
Randy, this is a copy of an email I received after posting about sand blasting. I thought it sounded pretty reasonable, what do you think?
Sandblast warping Sandblast warping
XXXXX XXXX here - I read your post / recommendations against sandblasting body parts and I am in complete agreement. However, the mechanism that creates warpage is not the heat associated with friction of the abrasives hitting the metal. Rather it is the compressive forces induced within the sheet of metal by the heavy abrasive particles hitting the metal surface. You will not see this same warping when light abrasives are used because they lack the kinetic energy required to compress the molecules in the steel sheet thus leaving the sheet in the same stress state as it was in when formed by the dies at the factory.
This is a common misunderstanding, yet this behavior is well understood by some industries and is exploited by those who understand it to improve fatigue life of certain parts (like connecting rods for instance - they are often shot peened to redistribute stresses and place the surface of the rods in compression to reduce fatigue crack propagation).
When you blast one side of sheet metal, that side of the sheet metal surface will build compressive stresses and will physically grow - this places the opposite side of the sheet into tension - this will result in the side being blasted to bow up toward the blasting source.
In order for warping to occur due to thermal conditioning - the metal actually has to be heated up to a point where the molecules reach a transition state where they go from one packing arrangement to another (close cubic packed to close hex packed) - picture a six pack of beer held together with the plastic rings - that is stress relieved / annealed steel. Now - take the cans out and stack them so that each row of cans is offset by half a can diameter - this is a tighter pack - although less friendly from a packaging standpoint at the grocery store. This new denser orientation is the orientation that steel molecules take on when raised to a sufficient temperature (depends on carbon content and alloys) . . . if you raise steel to this temperature and then quench it - you lock the close packing orientation into this dense packing structure and this results in shrinkage (and increased hardness / brittleness).
Using this technique it is possible to take a piece of 1013 mild steel, whack a piece off, heat the piece up and then drop it in a bucket of water, use a water cooled grinder to sharpen it . . . and then you can cut the parent piece of metal with it as it is harder and denser in this condition.
Soooooo . . . . that is the difference between warping / shrinkage of metal due to sandblasting (compressive mechanism - localized molecular packing) or due to thermal influences (gross - macroscopic transition of molecules to a higher density due to re-orientation of molecular packing).
With all this knowledge in hand (or mind) . . . you now also know that thermal conditioning is typically used to shrink metal on a localized basis where sandblasting will always cause metal to grow on a localized basis.
Hope this makes sense . . . thanks for the great writeup!
Jessie, I can attest this damage is not limited to late model cars. I understand it was a large flat panel like a late model, but the "good old steel" will do it just the same. I had a 1928 Buick door and a pair of 32 Ford doors RUINED by a sandblaster. I KNEW not to sandblast these parts to "white steel" but I figured a "dusting" wouldn't hurt. I was doing them for a display at a convention (ala NACE) for NAPA auto parts. The Buick door was SUPER straight and I was just going to have it lightly blasted, I would apply a coat of polyester primer, sand it and paint it. Then have a NAPA logo from the 1930s put on it like it was from an old delivery truck, cute idea huh?
So I took it to a sandblaster that I have had do a lot of work for me and my brother over the years. He did the 1922 Buick Roadster body on my brothers car, MANY frames, inner fenders, etc. over the years, he "seemed" to know what he was doing.
He barely blasted this thing, 90% of the surface rust was still there, he didn't bare down on it at all.
The door was TOTALLY RUINED (so were the duece doors but they were junk to begin with) I REALLY wanted to do this display and it was the only door I had so I had to "fix" it. I shrank it with a torch and a ROSE BUD tip! It was sooooooo screwed up it took me hours of work to "fix". I say "fix" because after I got it so it wouldn't oilcan I went to my "stock" of filler and mixed up a gallon for it! It ended up being DAMN straight, looked pretty good actually. It got a good coat of polyester primer, sanded and shot with an SS urethane bright red and the NAPA logo. It still hangs in the office of one of my favorite customers. It weighed a lot more than original, hope it doesn't fall on someone.
|11-26-2004 08:36 AM|
The biggest difference as to how the panel will react to being sand blasted is the amount of compound curvature present,
'Tight' panels with a lot of curvature, like '30s and '40s fat fenders, and inner panels that have strengthening ribs, will usually not exhibit any noticeable distortion after blasting.
But the large low crown exterior panels common to later model cars can be warped in a matter of seconds, I managed to destroy 2 nice 1960 Chevy hoods in just lightly sandblasting a few freckles of rust off the inside, after that frustrating experience I now only use careful hand sanding to remove rust from low crown panels, this is especially important if there is any inner bracing of the panel that cannot be easily removed (like on a '60 chevy hood)
On 'tight' panels like my old '48 Studebaker truck blasting works fast and the metal can be cleaned to the point called 'metal in white' that is as clean as it will go, this however is NOT advisable, as such harsh blasting peens the metal and work hardens it, making it brittle and prone to developing stress cracks.
Because of this I do not use blasting as a means of paint removal, I first remove all paint with a chemical stripper then use the minimum amount of blasting that will remove the 'loose' rust, if it is absolutely necessary to remove more rust beyond this point I employ the DA , hand sanding or a chemical rust remover.
However as I posted in the thread 'RUST...' I prefer on most interior or hidden panels to leave a thin even film of rust and coat with a 'Moisture Cure' coating.
|11-26-2004 06:07 AM|
Thanks for putting this altogether for me. I now understand whats going on here and the heat kinda put it all together.
I'm a sand blasting fanatic, I keep a dozen ceramic tips of three different i/d's in the garage as back up at all times.
Over the years I have herd of panels being destroyed but I have never personally seen it.
May just be sand blasting with common sense? Like using a 25 cent car wash and flaking the clear off?
|11-25-2004 10:51 PM|
The velocity is less, but mostly, you've just been lucky!
|11-25-2004 05:59 PM|
|oldred||If its ok I would like to put in my 2 cents. Some time back I created one of Martinsr's totally ruined,throw it in the trash parts, a 64-1/2 mustang deck lid. It took a new lid to correct that little mistake and from then on I was a lot more careful. I was using a pressure blaster at 120 lbs. pressure and regular sand(for lack of a better description)and even though I moved quickly and did not hold the blast stream in one spot when I stopped to check my progress I was shocked at the amount of warpage! Now I try to avoid sand blasting but I have used a suction blaster at 90 psi with no problems so far. I am not saying that a suction blaster is ok and will not damage a panel but it does seem to me to be a lot safer if someone does decide to sand blast. Could it be that the sand velocity is lower in a suction type blaster?or have I just been lucky so far?|
|11-25-2004 05:02 PM|
I've been reluctant to jump in on this one.
Whether it's male ego, or what, I'm not sure, but we seem to have this "my way is best" theory that we just can't seem to get over.
Personally, I will not touch exterior body panels with a sandblaster. Inner structural pieces are usually heavier and have lots of offsets, etc. that stiffen them enough that any stretching is not noticeable.
The high pressure, (even at 100lb) from sandblasting can, AND WILL, cause the metal to distort to varying degrees. Sometimes you get by, other times, you're not so lucky. The best aproach is to stay away from it!! I've seen far more panel absolutely ruined by sandblasting than not. Most by DIY'ers using small hobby shop blasters. It's not a pretty sight, nor is it easy to repair.
As far as the metal being stretched only on the outer surface....hmmm....don't think so. Think of what happens to stretch the metal. It's been said that the heat of the process stretches the metal. We know this to be false, simply because heating metal causes it to shrink, not stretch. Besides that, metal needs to be heated in a localized area to about 300 degrees for this to happen. If sandblasting is getting the metal THAT hot, I've certainly never seen it. The principals are the same as if you took a hammer and wailed away at it. Only difference is the size of the hammer. The sand is like millions of tiny hammer blows per minute. The surface isn't all that's getting stretched, but rather the entire panel, throughout. I have no known proof, but I would venture to guess the surface is extremely hard and brittle. Ever notice how tough it is to get rid of the sandblasted texture with an 80 grit DA pad??
Lastly, my advice, without seeing pictures, would be to find another set of doors and leave the sandblasting to heavier operations. It was never meant to be an option for cleaning automotive body panels.
If you want to safely remove paint and rust, your best solution is to have them professionaly dipped by a company worthy of such a task. Redi Strip is the way to go, as they do not use an acid based agent to remove rust and paint. They use a heavy alkaline solution that doesn't re-activate once moisture becomes present.
Good Luck with your project. Worse case, I guess I'll have to make you a new set of door skins!!
|11-25-2004 02:01 PM|
We are not in any disagreement I assure you. It can be done, it is done, I know that. My point maybe wasn't made clear. It CAN cause this damage and many times will.
I have seen little home sandblasters do it, of course how big these "little" sandblasters and or the compressor used is the issue. The fact is, I have seen it MANY times. You have either been lucky or used great care with common sense (I choose the later). Not everyone has the skills to pull it off. For this reason I aways recommend against it.
The problem is as I said this causes some of the most severe damage I have ever seen. I have been around this stuff my whole life and I have had darn hard time doing ANYTHING with some of the panels I have tried to repair. I am sure someone of Randy's talents can repair it (I should say, "I assume", there are some things that are beyond hope) few home hobbyest with the hankering to get out and sand blast have the skills to repair the damage it can cause.
As far as the repair, "how to tell if it is the inside or outside that needs shrinking?" that is a darn good question and the hardest part. How can you tell if both sides were blasted? If only the outside was blasted then the outside is what is stretched. If both sides were blasted, then both sides are stretched at different places? I don't know how you can find where to do the shrinking. I can do wonders with a crash damaged panel, but the sand blasting damage I have seen is just darn hard to repair. I have usually ended up "getting it close" and stopping it from "oil canning" and then filling the darn thing body filler.
|11-25-2004 12:02 PM|
I do have to agree, that is will take some work to get that panel straight now that it is stretched from blasting, since it is really hard to tell how much (area wise) it is stretched. Unfortunately, most blasting shields provide very limited viewing. That means that you can barely see what is being removed, let alone, if the metal is being stretched, or how much. Chances are that the operator never saw what he had done, until he had effected a large area. Unlike what we see in the collision work, where it is a small area that is stretched, and you can look and see what part is stretched, this may be an an area where "trial and error" might be the only chance. One thing about it. There is nothing to loose by trying/practicing.
Maybe Randy Ferguson has some tips that would help.
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