Filler over Epoxy Primer

[rick71ghia]

I read the article on this site by Serge on Body Filler Adhesion (very informative - thanks) and have a few questions. I'm using Glasurit 801-1552 epoxy primer. The TDS for the primer does not mention anything about a recoat window. Assuming that I prime the car and then come back anywhere from a few days to a few weeks later and apply Evercoat Rage Xtreme filler, how should I prep the primer? Just apply filler to primer? Sand the primer with P80 or some other grit before applying filler? If I sand the area to be filled to bare metal, any problem if I feather the filler into the primer around the repair area? How would the primer be prepped in this case?

[Serge]

I will tell you what I do...others might have different opinions...

I prefer to use filler on metal, bare metal that is...then epoxy on top of everything...so I would sand the areas that need filler to bare metal using 80 then do the metal. I would not feather onto the epoxy, I would for the sakes of having an even substrate under the filler, feather into metal....

[bodymanhelper]

I agree with serge and thats how we do it at the shop. Bodyfiller always gose over clean bare metal. I put 2 coats depending on what im working on then i cut the skin with 40 on a D. A. and block the rest with 80.

[fireboat]

Serge and Bodyman, I know you guys know of course that you are going against the tide as many tech sheets say epoxy first, then filler, then epoxy again.

That said, I always found that method odd because in fiberglass boat repair, the cardinal rule is epoxy over epoxy or polyester (auto filler is polyester resin) but NEVER polyester over epoxy because of adhesion problems.

Yet, with most autobody repair, it is done the opposite,...polyester over epoxy. Heck, I do it this way because I am just an amateur home painter and this is what most everyone says to do. But, I still find it troubling because of my experience with fiberglass boats.

I guess the bottom line is that both ways work and there are pros and cons either way.

[Serge]

Show me one tech sheet from a filler mfgr that says to put their product over epoxy...Not epoxy mfgrs who want to sell more epoxy...but filler mfgrs that don't want their products to fail...

They all say the same thing...over bare metal...Did you see my test on this site? I was expecting better performance from the epoxy to filler bond...The metal to filler bond was flawless though...

I would be interested in seeing the tech sheet you are talking about. Can you post a link?

[Len]

I had an argument with a paint salesman on the Hotrodders site about this. The question I asked that pretty much ended the discussion was.... OK if I apply filler over epoxy then sand the filler do I need to apply MORE epoxy over the bare metal before I apply more filler?

This application of epoxy before filler is a selling aid by paint companies to sell more primer. Like Serge said, you won't find a filler company telling you to use primer under their filler because they would sell less filler, it's all paint company talk.

With all that said, we prime metal that we strip if it's going to be undergoing repairs for a longer period of time. If it's a quick repair we don't prime. If the surface damage is very minor we may scuff the primer with 80 then fill over it but with a surface that needs any significant amount of filler we remove the primer, do the filler work then re-prime over the filler.

[rick71ghia]

Priming if the work is going to sit for awhile and then striping the primer in the damaged area makes sense. With two pros recommending it, that's what I'll do. Thanks.

[nickp]

I think I read (on this site) that Len did a demonstration/comparison of filler on direct metal vs. filler on epoxy.

the two sheets of metal were prepared and then he attempted to separate the filler from the metal via banging the metal sheet. The end result was the filler adhesion to bare metal was stronger than to epoxy.

this is on the site here, somewhere and this data is hard evidence that I'm going by.

maybe looking this up on the site will help out those who have questions. it helped for me.

NP

[fireboat]

Serge, I should have clarified, I was talking about paint tech sheets.
Of course, filler instructions are for DTM which I agree, seems to be the most correct way for best adhesion.
I say most correct (IMO as a home amateur) because sometimes it may take me a long time before I have the time to complete the metal and filler work and finally put epoxy on it. In that case, it is easier to put some epoxy on it first and then I can take my time doing the filler.
From my limited knowledge, is this as good? No, but it still seems to be a proven and satisfactory method.

[Len]

I guess I should have posted this link before now....

Link to Serge's Filler Over Epoxy Test

[fireboat]

Len, I have read Serge's experiment before. I think it was a relatively good experiment and I appreciate his efforts.
But, in the context of real world performance, I don't think anyone is going to see an adhesion failure either way. Over at Hotrodder's, someone did a poll and 75% said they put filler over epoxy.
However, I agree with Serge, I absolutely believe that filler over metal is the best method (again my disclaimer, I am just a rank amateur) and I will try to do it that way when I can, but I'm not going to lose any sleep by putting epoxy on first if the circumstances are appropriate.
I think we would have heard of numerous adhesion failures if filler over epoxy didn't work. Like many other things in life, there are differing methodologies that ultimately yield similar results.

[Serge]

The question is realy what is the BEST method...

Try a few coupons like I did, you will actually be surprised how easily the filler can be detached from the epoxy. Whey you look at the piece of filler you just removed from the coupon you will find that the epoxy has failed, not by poor adhesion, but by shearing in two...

Epoxy primer has alot of filler in it, it does not have a very high epoxy resin content...unlike the filler which has a high resin content, making it very resistant to shear and breakage...So, when used as an interface coat, although it could be sufficient, it will be the weak link in the repair.

The bond of the filler directly onto the metal is so strong, its almost impossible to shear it off the metal...

Now, take your entire coating in consideration. Would you like to have a hard mass island like a filler repair, floating in easily sheared paint? In fact, if done properly, the filler over epoxy actually floats the filler in an epoxy shell, it might be called epoxy, but don't be fooled by the name, it is still just paint and has little actual epoxy resin in it...Not good...in my book anyway...

The only time it can help, is if your repair is porous and you know there will be some moisture comming from the back side of the repair to get the filler wet...but in this case it's just a bandaid on a bad repair and it will fail eventually anyway...

My opinion is based on experimentation...experiment with it and you too will be convinced...

[Len]

The part I don't understand is that putting epoxy under filler you would spray the primer on then apply your filler on top then sand the filler which exposes the metal around the filler. So if you need more filler (which is almost always the case) do you apply more primer or does that filler not get primer?

Next time a paint salesman tells you to apply primer under your filler ask him about the second coat of filler and see what he says, I'd be interested in the answer.

You won't find many professionals using primer under filler, even the high-end shops rarely use this method and when they prime before applying filler they usually grind off the primer THEN apply the filler.

If you want the filler to bond well with the surface then push your first pass into the surface scratches in different directions then build on top. With each additional application you should do the same thing, push the first pass into the surface texture (this is your primer coat) then lay your heavier passes on top. If you're using a good filler on a properly textured surface and a good application technique you won't have adhesion problems.

[Serge]

Its impossible to block down filler without compromizing the filler around it...Imagine the mess created by putting a coat of primer between every coat of filler...you end up with a fragile sandwitch of primer/filler/primer/filler/primer...

That is completely ridiculous...

[55chevrolet]

Serge's test was not scientific enough to satisfy me.

Why do BMW and Porsche REQUIRE epoxy before filler on thier warranty work?

Is it to sell more Primer?

mmmm...