Hot Rod Forum banner

Sanding "bondo" with 40 or 36 a No-No!

29K views 44 replies 19 participants last post by  evilone 
#1 ·
Well, I received a shock the other day that blew me away. I was telling my employer about the method for filler work I was teaching an aprentice and he said "That is your first mistake, that is not company policy" HUH? What in the living hell are you talking about? This is what I was thinking of course, not actually saying :).

He says that you are NEVER to sand plastic filler with anything coarser than 80 grit paper. Even if you are going to apply more filler over it! This was taught to him he told me in an ICAR class or something on using body fillers. And not just one class but many. I have been to a heck of a lot of classes and never heard this, or at least I didn't want to. :)

The reason being that if you leave these 40 grit scratches in the filler when you apply the next coat of filler the scratches are so deep that the filler won't go down to the bottom of the scratch. This will leave a little pocket of air at the bottom of the scratch that will expand or and cause swelling, or shrink down as more solvents are added over the top in the primers and paints.

Now, this IS the reason for sand scratch swelling or shrinking with primers and paints, but I had never heard it being an issue with plastic fillerers which just don't shrink to any degree.

Why in hell we have 40 grit paper in the shop is beyond me if he didn't want us to use it. 8" for the killer, 8" for the hog, long board, stick-it non stick-it, we had it for the DA, for every tool we use, we had 40. Ok boss, we won't use 40 anymore. I put a note up for the salemen from the paint store DO NOT stock 40 in anything any longer.

The last shop I worked at used nothing coarser than 80 and we got along fine so I thought, no problem, he's the boss, no 40, done deal. Another tech went nuts so I went ahead and called Evercoat tech, sure enough, they say not to use anything coarser than 80, for the reasons he laid out.

So guys, if you want to go by the manufacturers recommendations.....nothing coarser than 80. I hardly ever use the 40, but on those quarter panel length filler jobs it will be sorely missed by this guy. :(

Brian
 
See less See more
#5 ·
Shine, exactly.
Is all the work I've done the last 25 yrs with 40 grit now going
to have a problem? (LOL)
At the very least, if it's a concerne I'll sand with 40 grit and once
shaped maybe go over it with 80 to rid the scratches,
That would take all of two minutes. :pimp:
 
#6 ·
jcclark said:
Shine, exactly.
Is all the work I've done the last 25 yrs with 40 grit now going
to have a problem? (LOL)
At the very least, if it's a concerne I'll sand with 40 grit and once
shaped maybe go over it with 80 to rid the scratches,
That would take all of two minutes. :pimp:
That is exactly what I feel, but the boss has said "NO 40 grit", ok, no 40 grit.

The way Evercoat looks at it is simple, like a lot of companies, they make a blanket recommendation to eliminate any mistakes in application.

Obviously, if you sand out the 40 grit scratches with 80, it's all good. HOWEVER........if the guy doesn't sand them all out completely, you have the same conditions they are concerned with.

It is like etch primer, S-W says you can apply it over "cured" plastic filler. They are the only company I know of that states this. They have given the go to the tech who will listen and make sure the plastic is cured. Other companies say NO application over plastic filler. Why, because they KNOW sure as hell that guys will apply it over uncured half hour old plastic filler and there could be a problem, which will become THEIR problem. So.....if you apply it over filler and have a failure, there is no "it was cured, honest", the manufacturer is not going to back it.

It was funny, because when he explained it to me (complete with diagram in dust on the hood) I said "funny you should say that, because I specifically instructed him (the apprentice) to press the polyester putty down hard into the scratches and pin holes on the first pass, then apply more with less pressure to get the even skim coat over the whole thing".

But, he hands me my check every friday, a nice fat check I may add, so I will do as he says, no questions asked.

Brian
 
#9 ·
Boy oh boy, I bet the boss would really freak out if there was a cheese grater type file in the shop! Honestly, I can see where the problems might be with using 36 then not getting a good tight second coat but if the user is cutting it down even slightly with 80 grit before spreading another coat there shouldn't be a problem. At one time I thought anyone using 36 or 40 for shaping filler work must be a hack and refused to use it, for years all my filler work started with 80 grit or finer. Nowadays I keep 40 grit in the shop both in board paper and 6" and 8" DA sizes and use it with common sense. And yeah, I also use a cheese grater now and then for rough shaping fast with no problems.
 
#10 ·
Brian, you're absolutely right, it's always better to
error on the side of safety.
I'm sure a lot of things mfg's warn against are only
because of the conditions that might not be adhered to,
I understand that. They want "fail safe" procedures, I
don't blame them. People abuse every rule.
But like Bondoking said about my
"It's always worked for me mentallity"
Well, I am stubborn (LOL)
Thanks for the article, I will be more careful now about
my scratches but I need the 40 grit to straighten it initially. :pimp:
 
#12 ·
I'd be kicking and screaming if I was told I couldn't use 36 grit. specially some of the places I worked were entire panels were filled sometimes, or a boat builder with fabricated parts that had a lot of area to fill. I was taught to rough shape with 36 grit, and every shop I worked in used it on anything other then small areas. I would be very unhappy if I had to cut down bodyfiller starting with 80 or finer and get it straight without spending an eternity on it. I also have a much easier time getting something straight with a course grit then finer ones. If someone would have told me that I would think they were nuts because I haven't seen a problem starting filler sanding with 36 and sandscratches because of it, if finished off with finer grit and skim coats, sanding and repriming, and allowing primer to sit. Is this perhaps something fairly new Icar came out with? Back in tech school we had documents from I car and never once read or was told anything like that, but back then people were first starting to apply fillers over epoxy, with maybe a little more places applying over baremetal, probably afraid to change, but a few I worked in around that time did use epoxy first, and they were more quality oriented shops.
 
#13 ·
jcclark said:
People abuse every rule.
But like Bondoking said about my
"It's always worked for me mentallity"
:pimp:
Jim that was a blanket statement, not directed to you Ole buddy... It is a common thread that links most of us together... we are hard headed and almost refuse to change any of our methods...Probably all the fumes we inhale :pimp:
 
#14 ·
I very rarely use 40 grit paper to sand filler. The only time I use it is when I forget about my mud and its super rock hard or doing one of those full quarter panel mud jobs. Even then I only use it to knock it down. Once its cut I move over to 80 grit.
I see NO problem using 40 grit THEN cutting it with 80 before you apply your next coat.
 
#15 ·
40 grit is ALWAYS a source for swelling and shrinking. Some guys use it exclusively though I'll never understand why. How about get it closer so you don't need as much filler? 40 then 80? Maybe. I hate filler. I see lots of guys that think "it's enough" but not really. You gotta be thorough.


" Now, this IS the reason for sand scratch swelling or shrinking with primers and paints, but I had never heard it being an issue with plastic fillerers which just don't shrink to any degree."

Brian you're a talented and qualified guy in this field. That statement surprises the schitt outta me coming from you. Bondo does indeed shrink. Maybe not a bunch but it does shrink and that condition is aggrevated even more when it's freshly done then primed over. A necessary evil, no doubt, but it happens. Try a fill, block it to perfection, then check it the next morning. Is it terrible? No. But it's there bro. The thicker the mud the more the shrinkage. That freakin "halo" around a repair. I hate filler.
 
#16 ·
when I did the bondo work on my '51 Chevy, I started the sanding with a mouse electric sander and 220 paper, 2 days and did not touch it, got some 100 grit for it, still no dice, 80 grit on a peice of 2x4, worked ok, still not fast enough, got some 40 grit, put it on the 2x4 and it was str8 in an hour or so, used some 80 on it to smooth it out a little and it's all good now.
 
#17 ·
Yes,, some of us are hard headed, and unwilling to change. One thing that amazes me is that the products are supposed to be gettig better. Yet, as time goes by, they decide that the methods that we used to use are no good. They worked just fine with the products that were out before they were "improved".

Aaron
 
#18 ·
theHIGHLANDER said:
" Now, this IS the reason for sand scratch swelling or shrinking with primers and paints, but I had never heard it being an issue with plastic fillerers which just don't shrink to any degree."

Brian you're a talented and qualified guy in this field. That statement surprises the schitt outta me coming from you. Bondo does indeed shrink. Maybe not a bunch but it does shrink and that condition is aggrevated even more when it's freshly done then primed over. A necessary evil, no doubt, but it happens. Try a fill, block it to perfection, then check it the next morning. Is it terrible? No. But it's there bro. The thicker the mud the more the shrinkage. That freakin "halo" around a repair. I hate filler.
If you take my quote at face value, I did say that filler doesn't "shrink to any degree". That "degree" is up to interpretation. Your "maybe not a bunch" sounds a lot like my "to any degree", both sound a lot like "a little", "a smidge", "a trifle bit" "imperceptible, or "a **** hair" :)

Personally, I think the difference you feel the next morning is the fact that your hands, mind and preception are "sharp" after you have left it alone and had a nights rest. It felt the same way the night before, you were just too burnt out from sanding and feeling it to pick out the flaws.

To get back to the whole issue and why Evercoat recommends the 80 or finer makes sense. If one were to block something flat with 40 and then put a skim coat of polyester putty over it, sand it flat just barely hitting the the underlying filler one would end up with these scratches under the polyester putty with only the polyester IN the scratch. This "could" open the door for a problem if the putty wasn't pressed down into the scratch well. Lets face it, there are a LOT of guys who just don't "get it". I have a few where I work, they really don't "get it". They could easily end up with a deep scratch that is not filled with the putty and that air pocket in the scratch would do EXACTLY as Evercoat describes as a reason for the recommendation.

Like Jeff Foxworthy says in refering to the doctor telling he and the wife to not have sex after the water breaks... "They wouldn't tell you not to do it if someone hadn't done it before".

"What do you think honey, the contractions are fifteen minutes apart, we can get a quickie in" :)

Brian
 
#19 ·
I wouldn't work at a place that would enforce a ridiculous rule like "no 40 grit, ever". Any place that feels like a rule like that is necessary must have a few bad apples that are causing comebacks, or a egomaniacal know-it-all (expletive deleted) for a boss.

Easy way to cure the problem is to make sure that the surface is "in 80" before reapplying filler, meaning ALL 40 scatches are gone, even if it means the surface goes a bit too low. I go a step further and make sure that anywhere the filler will come up to or overrun a featheredge, the featheredge is in 150-180 grit.

THAT is where the real problems come in, IMHO, is when guys will skim over their nasty ragged featheredge job with filler, block it smooth, but you can see where the filler is inside some preety nasty scratches in the paint. Wanna talk about shrinkage and edge mapping? Ugh.

Anyway, for a job that is over say a square foot or so of filler coverage on a heavy fill, using 80 to break the surface and remove spreader marks makes no frickin sense to me. Why instantly clog up a perfectly good piece of sandpaper, and the job goes WAY faster and comes out straighter if a guy starts with 40.

My 2¢, to each their own!
 
#20 ·
Since we are on the subject, what is technically cured filler?? If it shrank so badly, collision shops would go under as filler is applied, sanded, primed and painted all in same day, and these vehicles live out the remainder of their life with the repair undectable... well most of them if they are done in quality shops that is.. so again, where is the gage here??

Brain, Bob, Jimc, Barryk, what are your thoughts?? The polyester prince does not see the shrinkage issue that was once common with cheap fillers of years ago.. todays quality fillers and glazes are far better than they were just 10 years ago.
 
#21 ·
Polyesters shrink for sure, it'll pull up and bowl shape a piece of 1/8" hardboard if continually used as a mixing board. When is the shrinking done? My guess would be when there's no more fumes coming out and that'll depend on temperature and mix ratio. I'd bet on average that the shrinking is mostly complete within a few hours, and totally done within 12 hours. Just a guess though. How much does it shrink? 2%? 5%? Bob
 
#22 ·
Bob, that shinking that is bending your board is done in the first few minutes as far as I am concerned. So, after it is sanded the shinking is all but done. 12 hours is "fully cured" as far as S-W is concerned.

Of course, a little shrinking isn't going to cause a whole lot of complaints doing collision work. Heck, you can send some pretty pee poor body work out the door, most people just don't see it. NO, that is now how I do business, but it happens. I just don't think it shrinks to ANY "Measurable" degree.

Crashtech, first, we just don't use a whole lot of filler, I replace a heck of a lot more than I repair. And HE pays for the sand paper. So, yes I do change the paper often, VERY often. But I have always done that, I want my paper to CUT, not polish the surface. Paper is a LOT cheaper than labor, he doesn't have a problem with that.

I get paid well, if he doesn't want me to use 40, I could care less.

Brian
 
#25 ·
Just now got a chance to read this.

This crap of sanding filler with 80 and finishing with 180 started in the mid
80's when some of the euro companies started selling here because there Idea back then of a primer was slower and thinner than Americans primer.

They since have changed their products.

As far as I-car I would take anything they say with a grain of salt, it is nothing more now than a big money making organization and most of the instructors are not qualified to do body or paint work but make a killing teaching the class.

Me, I will just keep working with my 40 air file and finishing with 80 grit.
Guess I will never get a Icar certificate for the wall.

Oh, in-case there are any I car instructors on here, I will give you more info to flame me.
Metal work 5" and 3" 24 grit.
Filler 40 and 80.
First primer blocking 180. (polyester I use 100)
Second primer blocking 180.
Final primer sanding 320.
 
#26 · (Edited)
How to "treat deep scratches" is really the original posts point. How do you treat the scratches. Evercoat says "Don't put them in to begin with and you won't have a problem". Ok, realistically I think they are going overboard with the recommendation to not use 40/36 at all. However, it does do the trick, so they aren't so dumb.

The fact is, these scratches as others have pointed out CAN cause you trouble if not dealt with properly. I work so hard at not leaving any exposed scratches, I know (think I know) I take care of it. But I also know that I have left an open door for the problem to raise it's head when I sand the skim coat down to that transparent state over the filler that may have been cut with 40. I have had in my thoughts that I could cut that filler down with 40, then apply a skim coat of polyester putty over it the putty WILL certainly fill the scratches. Well, they will..IF...it is pressed down good into them. What if I slip up a little when I spread it and a scratch doesn't get filled well, it leaves that void at the bottom of the scratch? After it is sanded to that transparent look right in that same spot.....that scratch is coming back to haught me.

Just a little tiny change in my procedure and it is fixed, for ever. I think that is a pretty good solution to the problem. It isn't a change I need to perfect, practice, nothing. Just simply stop using the coarser paper, done, that is all there is to it. Seems like a pretty good solution. ESPECIALLY with the "other" guys, you know those other guys, the guys who do all the bad work. :) None of US would make a mistake because we are sooooooo skilled and conscious. I mean, WE wouldn't make such a mistake, but we know someone who would. :)

This is one of the reasons behind my "Basics of Basics" skim coat, this very reason, to ELIMINATE all chances of scratches for the primer fill.

As Crashtech pointed out, just plain leaving the scratches there and not sanding them out is one of the biggest causes for a scratch problem. When I do filler work, I don't EVER try to "save" a filler application so I don't have to do a skim coat. I don't apply the first coat of rage and after getting into it a little think "Hey, this is all I will need". I ALWAYS cut it flat and then apply a skim coat of polyester putty as sort of a "primer". Then sand that with 180 and it is FA-LAT and smooth, ZERO scratches for the primer to fill.

I work with 7 other bodymen, most don't agree with my method, some have "seen the light". It works, it simply works. It is not "my method", it is Emery Robinsons method, the guy who taught me 25 years ago.

I watch the other guys try to "save" that first application. They will switch to finer paper WAY too soon to do this. They try to "save" every single application, they may apply three, four, five, even six or more, each one, in their head is the "last" coat!

And when they are done, they have metal popping up here and there, they have "patched" it here and there, all in an attempt to "save" the thing.

THEY end up with scratches as Crashtech described. When they sand down with the 40 or even 80 and some metal starts showing where it is likely a little high, they switch to 120 or 180 and then start the CPR to save the application. Well, there are scratches that went into the filler that are now below the metal height that popped up. How in the living hell can you block it all smooth if you have metal popping up and STILL have scratches that are lower than the metal? THIS is how most scratches end up staying.

They may apply a little polyester putty over that area, then try to block it all out and still, some of the original metal that they ground with a friggin roto-tiller is popping up! THAT metal is full of scratches that the sand paper isn't sanding out. Again, more scratches for the primer to fill. Plus, the different layers of filler here and there, it looks like a friggin patch work quilt, the primer will have to level that all off, goodness knows it can't be perfectly flat going from patch to patch of filler or putty.

I CUT the filler flat with the coarser paper (80 now :)) and if any metal "pops" up being too high, I tap it down then. The filler has the shape of the panel, but not to perfection. It is all a little low as Crashtech pointed out, we are talking "a little", "a smidge", "a trifle" low. A skim coat of polyester putty over the whole thing, going out onto the surrounding straight metal a good distance is applied. ALL the scratches are covered with this "primer coat" of polyester putty.

THAT is blocked with my DynaBrade orbital "DA" with 120. That usually leaves it pretty nice, but before it is completly flat, again, it is now "a smidge", "a trifle" high, we are talking 120 grit scratch depth high, I switch to 180 and after dry guide coat block it to perfection.

The ENTIRE area is covered with polyester putty as a "primer", I MAY hit a little filler or metal but just barely, it is all in that "transparent" look.

I do filler in two coats, period. One in Rage and the second in Glaze coat. Probably 95% of the time, two coats. Yes, some may have been done in one, but 95% in two and the 5% in three (or more if I am having an off day) makes more sense to me than trying to "save" every friggin coat and having 95% with three or four applications and being sent back from the paint shop needing more work because it isn't good enough just to get 5% with the one coat.

Red, I just ruined this tread with a "how to" didn't I. :) Sorry.

Brian
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top