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-   -   preparation of pitted metal after shotblasting (http://www.hotrodders.com/forum/preparation-pitted-metal-after-shotblasting-56776.html)

rlackey 01-26-2005 03:38 AM

preparation of pitted metal after shotblasting
 
I am about to have the body of my '54 Chevy blasted and have a few questions about preparation for primer afterwards.

First off, the body is covered in surface rust, and there is not much paint left to speak of. It has been sitting in a field for many many years. However, the body is very straight apart from the roof (see thread: Repair of large dents in roof?) and looks as though I should be able to go straight to primer after blasting.

The surface of the metal is pitted quite badly, but to my untrained eye there seems to be adequate metal thickness, it looks and feels very solid. My first question is how do I gauge that that there is enough metal there to prime and paint, I am inexperienced and won't just trust that it "looks and feels" solid, there must be some kind of guideline I can go by.

My second question has more to do with the surface condition of the metal. The shop that will be shotblasting it says that the process leaves a very good surface for priming (he quoted some measurement in microns, I can't remember the number). However, as I mentioned, the surface is pitted quite badly to begin with and I imagine that even after blasting, it will be rust free, but still pitted, perhaps not a suface suitable directly for priming.

I imagine I could smooth it out with a very thin skim coat of filler, but there has to be a better and more professional way to get a smooth surface suitable for priming on rust damaged metal.

I could also sand it smooth, but am afraid of loosing even more metal thickness.

I have read a lot of the very good FAQ's and so forth on priming and painting and the preparation necessary but have read nothing that deals directly with badly pitted metal. They seem to assume a panel that is somewhat smooth to begin with, at least smoother than mine.

Any help and expertise would be greatly appreciated.

astroracer 01-26-2005 03:53 AM

I would suggest doing this panel by panel as much as possible. This will give you the opportunity to get the panel primed as soon as it comes home from the blaster. You will get many opinions from guys on the board and some of them may think my process is stupid, but this is what I would do.
1). Get the panel home and immediately wash it down with Duponts' Metal Etch.
2). Wipe it down with Prep-Sol and spray on a coat of Dupont Vari-Prime self etching primer. This will bite into the base metal and provide a good base for:
3). A couple heavy coats of epoxy primer.
4). This combo will protect the parent metal and give you a perfect base for applying primer surfacer that you will block down to smooth over the pits.
You will want to pay attention to the inside of the panels also.
I have used this process for years and had very good results with no quality issues.
Mark

rlackey 01-26-2005 04:50 AM

astroracer

Thanks for the advice, I'll see what others say also, but that sounds good to me. I was planning to do it panel by panel anyway... fenders, hood and trunk lid, doors, with the bodyshell itself last.

What would be your recomended treatment to protect the inside of the panels? I assume I should also get the inside surface blasted as well?

MARTINSR 01-26-2005 08:11 AM

Do yourself a favor and go to this thread "The secrets of surface rust removal" (click here) .

FORGET ABOUT ANY SORT OF "SAND BLASTING". I know of no "media" (the material you could use to blast with) that will remove RUST that isn't dangerous to use. There are all kinds of medias that you can safetly remove PAINT like "plastic" (little plastic balls) , "Soda" (soda ash, "Walnut shells" (crushed walnut shells) and others. But anything that is actually going to remove rust has to be a little heavier like sand. Sand, ANY sand can and many times WILL cause irrepairable (to most mere mortals) damage to sheet metal.

If you think you have a challenge right now with that roof panel, you don't know you are alive. Sand blasting damage is like nothing else, it isn't "Darn I messed up, I have more work to do" kinda damage, it is "#@&* I can't believe I &*%$#@ up the panels this bad they are totally RUINED and I have to throw them away!" kinda damage.

The sand particles act like little hammers and spread the molecules on the surface of the metal apart stretching it out.

If I took the time right now to search out threads on this subject, people who came for help because their car or panel was TOTALLY DESTROYED this post would be a foot long with links.

YES, it "can" be done, it is done everyday. HOWEVER, you just don't know who you are dealing with, I personally have had an "expericanced" sand blaster RUIN a couple of doors for me. The funny thing was, I didn't care. To make a long story short it was a project that didn't have to be "perfect" so I took the chance. Well, he delivered them back FAR from "less than perfect", they were almost totally destroyed. And if you think YOU can do it, sure, maybe, but more likely you will learn how NOT to do it by DESTROYING something you treasure.

Here is just one of those threads that was here on Hotrodder a few weeks ago Click here .

Randy Ferguson 01-26-2005 12:05 PM

Take Brian's (MARTINSR) advice!!!

Sandblasting sheet metal should only be done in a very confined area of a panel, when there is no other option to get it clean. Sandblasting complete panels, let alone an entire car should never be considered. The risk is far to great. In my opinion, it will also harden the metal, making it brittle and prone to form fatigue cracks easily.

Also, astroracer, you might check with DuPont to see what they suggest for sandblasted matal. I think you will find that they recommend NOT using a metal prep, NOR acid etch primer to sandblasted metal. There is a possibility of acid entrapment that will cause major problems down the road. It's recommended to thoroghly sand the metal smooth before applying either of these products.

Epoxy straight to bare metal is far better than using an acid based etch primer.

Hopefully Barry K will enlighten us more on this subject.

When you get your surface rust removed, coat everything inside and out with a couple generous coats of epoxy primer. I prefer Southern Polyurethanes, Inc. epoxy over any I ever tried.
www.southernpolyurethanes.com

After applying the two coats of epoxy wait an hour or so and start applying high build urethane primer. The epoxy will give you the best adhesion and corrosion resistance qualities possible. The high build primer will fill the rust pits. It will probably take a couple applications of about 3 coats each of the high build primer, but in my opinion, it's the best approach. Some guys like Featherfill type products, which is basically a polyester sprayable filler, but I've had nothing but bad luck with it.

Randy Ferguson

astroracer 01-26-2005 03:41 PM

Thanks Randy,
I'll take your word for it. I have done a half dozen bumpers using the exact process I outlined above with no returns. I used the metal etch to clean off just a hint of surface rust after the blasting process (which will not hurt a bumper if you are careful). Maybe I dodged a bullit because I cleaned up with Prep-Sol after the etch?
I am no pro at this by no means but at least you didn't say I was stupid. Thank you for that.
Mark

willys36@aol.com 01-26-2005 04:52 PM

Metal prep acids are phosphoric acid which converts rust to iron phosphate which the chemists tell us is a super good base for subsequent paint jobs. Who knows!? I agree with Randy though, the mechanical tooth of a sandblasted surface combined with the adhesion can't be beat by ANY system, period. Just too bad that sand blasting and sheet metal don't get along.

Check your yellow pages for a chemical stripper. They remove everything but the shine on the metal.

I had about the same surface as you describe on the '36 Pontiac that I rodded and I sanded the surface with 36 grit and my trusty Harbor Freight DA. Took a couple of days but do-able. That got the heavy rust off but there was still a hard iron oxide coating on the metal. I soaked this with one of the blue liquid phosphoric rust converters (Rust N-M-E brand as I recall) which turned it to black iron phosphate. Don't get this stuff on your cement garage floor. It eats it up QUICK! Three coats of polyester baby potty yellow primer followed. That isn't the most waterproof coating for sure but that finish went through wind, rain, and snow (not really - doesn't snow in Bakersfield) with nary a sign of rust. I installed a frenched radio antenna in the trunk hump a year or two later and had trouble sanding off the primer it was stuck so well and when I got it off, all I found was fresh steel, not a sign of rust.

rlackey 01-27-2005 02:20 AM

Ok guys, I get the idea about the sandblasting, or any type of media blasting for that matter... ok on cast iron exhaust manifolds, not ok on sheet metal. I'm just glad I didn't do it yet!

These cars are not easy to come by in South Africa (even though they were manufactured here... mine is a RHD 4dr Bel Air built in Port Elizabeth), but reproduction sheet metal is impossible to come by without importing from the U.S. (at great expense).

Therefore I cannot afford to ruin anything on this body as it's the only one I have. This is also the problem with the roof, it is badly damaged, not just dented but creased in places right to the drip rail. It's a mess. I have no choice but to repair it... this is actually my biggest worry at the moment, but not the subject being discussed here, although I would appreciate anyone's advice. Anyone fabricated thier own replacement roof? I have another thread on this problem, see "Repair of large dents in roof?" (not sure how to include a link directly to another thread yet).

MartinSR, thanks for the link to the thread on rust removal, I had read that thread before posting but did not know the dangers associated with media blasting so still assumed it was quicker and easier. One question regarding the process described in this thread, and I hope it doesn't sound stupid, what is naval jelly? I have never heard of it here in South Africa, maybe we call it something else.

So in summarizing am I right in thinking my safe options are basically down to chemical stripping or good old sand paper and elbow grease (or a DA sander), and then as Randy suggested a couple coats of epoxy primer right on the bare metal, followed by a high build urethane primer.

This being the case, I have more time than money to put into this project anyway so elbow grease it is.

I actually apologize for bringing up this whole very tired subject that has probably been covered a zillion times, but I am grateful for your expertise, you guys may have saved my car.

rlackey 01-27-2005 03:21 AM

Randy,

I just read though your guide on repairing the Willys roof! Amazing stuff but the pictures don't seem to show on the post now. Do you have it in some kind of document form or is there any way you could email it to me with the pictures if I give you my email address?

I have one heck of a job ahead of me on my roof!

Randy Ferguson 01-27-2005 07:25 AM

rlackey,

The pictures seem to be loading fine now. A little slow perhaps, but they are loading!! If you still can't get them, let me know and we'll work something out. It would be good if you could post pictures of your roof too.

As for the neval jelly, it's a mild phosphoric acid based rust remover in a "jelly" type substance, for lack of a better term.
It will stay on vertical panels and will also remain wetter longer than thin rust removers normally used. I've found that covering it with Saran Wrap, or whatever you have available as a thin plastic film, will also keep it wet longer, as it slows the evaporation rate.

I've used this trick for keeping paint stripper wet, so I tried it the other day with the naval jelly. It works!!

If you can't find the naval jelly there, get whatever type of rust remover you can and it will work. You may have to lay out a cloth on the surface and soak it with the product, but I imagine the plastic wrap would work on it as well. The trick is to keep the area wet at all times. Only work a small area at a time, rather than the entire panel, as this will help to keep it wet and it won't get away from you. Now, it's possible to work several panels at a time if you set up a production line.

Also, If the door jambs and such are rusted, you can get by with blasting those areas, just stay off the exterior body panels. You go around fender edges, trunk lid openings, etc. with the blaster. This will speed up the process.

Send us some pictures!!!

Randy Ferguson

rlackey 01-27-2005 07:53 AM

Randy,

Thanks for the explanation, I'll post some pictures just a soon as I figure out how.

The door jambs are actually in great shape, still have a decent coat of paint on them, so can probably get away without blasting at all.

Thanks again, I actually can't wait to get your advice on the roof, you are a very good authority to tell me if it's repairable or not as I actually may be making a mountain out of a molehill as such, just because I don't really know what can and can't be done.

Just something else quick, the only other damage I have to cut out and replace is the floorpan, both the rocker panels and the bottom door skin of the drivers side door (would be your passenger side I guess as this one is RHD), the other doors are very solid apart from the exterior surface rust.

As I don't have easy access to aftermarket replacement sheet metal in this country, will it be fairly easy for me to fabricate the floor, rocker panels and lower door skin from scratch?

I'll get those pictures up ASAP.

rlackey 01-28-2005 12:32 AM

1 Attachment(s)
Randy, here are a couple pictures of the roof damage. There are a few more pictures of the car as it is in my gallery.

Any suggestions you have would be very welcome.

rlackey 01-28-2005 12:33 AM

1 Attachment(s)
Oops, seems I can only attach one file at a time. Here's the other one.

Randy Ferguson 01-28-2005 07:55 PM

Mr. Lackey,

It looks bad..........BUT............I "think" it may not really be as bad as it appears. The first step would be to remove the headliner and evaluate the damage from inside. There's a good chance much of it will pop right up to where it belongs with a simple push from underneath. I wouldn't use a hammer to persuade it at this point, so don't get carried away!! Hammering on it will only worsen the condition, creating even more stretched metal and more work for you to repair. If you can't push it up by hand, you can use a porta power (if you have one) or simply use a bottle jack and a board. Something to help spread out the force over as large an area as possible would be a good plan as well. For the most part, what has happened, is the roof has been turned inside-out. There is n doubt some stretched metal as some point, but for now, let;s just concentrate on getting the bulk of it back to where it should be. Once it starts over center, it should pretty well pop right up to where it needs to be.
Getting into just a little metalshaping terminology here. Right now the shape (area) is correct, for the most part, but the form (arrangement) is way off. When shaping a piece of sheet metal, there are two basic things we read as the panel is shaped, that being area and arrangement. The area is the amount of stretch or shrinkage placed into the panel at the right spot to give it the desired final shape. The arrangement is simply bending and twisting it to the proper final form so that it takes on that desired look. This is what's going on with your panel.
The change in total surface area is only slightly compromised, meaning that it's not stretched all that much, however, the arrangement is WAY off.
To further explain this concept, let's take a shop rag for example. You can fold it, wad it, roll it etc., into about any form you want to put it in, but when you lay it out flat, it the same size it was when you started. You do this all day long, and there would be no change in the size of the shop rag. That's because you have done nothing to change the overall area of it.
Now, take that same shop rag and twist it as tight as you can, pull on it in several directions, tug on it for all you're worth!! Now what's happened??? When you lay it out flat again, it's larger than when you started. Why?? because the fibers have been stretched. The surface area has grown, rather than just being re-arranged!! The same happens with metal.
We all get a little freaked out over dents at times, but what it boils down to is learning how to read the metal, in order to tell it where to go. Overcoming the fear of failure is the #1 secret to success. You must always be in total control of the metal. YOU are it's master!! Never allow it to control you!!

Once you have the majority of the dents pushed up, I would suggest removing the rust. First you want to make sure the dents will come up as easily as I think they will before spending a lot of time removing rust. The rust will need to be removed before proceeding with the final dent repair though, as you don't want to hammer on rusty metal. It'll drive the rust into the good metal, making even deeper imperfections to deal with.

My apologies if I've gone deeper than you'd like, but I think it will help you understand it better once you really get going on it.

I'll need to look at the rockers and floorpans to give you sound advice on those. The floorpans and lower door skin should be fairly easy. I think the rockers shouldn't be too bad, but I haven't looked closely at one of these in a while.

Randy

rlackey 01-29-2005 10:00 AM

Randy,

Thanks so much for your input, it wasn't too in depth at all. I want to know what's going on and the best way to fix it is to know exactly what is wrong with it in the first place.

I hope you don't mind if I keep you posted with how it's coming along. Some step by step help might be what's going to get it back in shape.

Thanks again,

Rich


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