Wife,........."You drove how far for that thing?"
Daughter,..."Theres no inside and it stinks."
Friend,......."Dude, thats a rusted piece of sheet."
Son,.........."This old car is cool."
I will never put body filler on bare metal. First, moisture trapped in the air can be trapped on the surface and in the filler and condense on the metal when exposed to near freezing temperatures. Second, if there are pinholes or if the metal is significantly porous, moisture will wick into the filler. I always seal the metal surface with EP right after preparing the bare metal.
Based on what I have learned at places like autobodystore.com and talking to manufacturers, Either is correct and will work fine provided you follow the manufacturers directions.
In talking with the owner of SPI he recommends laying down epoxy to properly prepared metal first. Then during the recoat window layer in the filler. He explained that the surface of the epoxy will have small holes in it that close up as it cures. So the bondo has 'strands' that go into the holes and when the filler cures the epoxy will cure quicker from the heat and the holes will close up on the filler strands.
My understanding is that epoxy first then filler will give the best metal bond. Epoxy will also seal the surface from moisture. Now you will hear the people in business laying the filler first then the epoxy. Turns out that is more profitable in a production shop as it is less time consuming.
Keep in mind there is a lot of bad info on the proper use of both epoxy and fillers. I suggest that you pay attention to what the manufacturer finds works best. Improper surface prep is the most common problem. I know many people have this belief that a very course grit paper is was the filler needs. In reality it needs a lot of scratch per inch at the proper depth. This varies with the fillers as you can quickly find out by looking up a bunch of products from the same manufacturer.
My body guy and the local auto body supply house swear by epoxy on metal, then filler, then high build primer, then epoxy (or sealer) then paint. They tell me you want a sandwich (where epoxy is the bread, and filler the meat) This protects the filler and ensures no moisture ever gets to the bondo, (look at prev bondo jobs where it went on metal- see the rusty areas under the bondo? there is the issue) It will bond pretty good either way, but bondo popping loose seems to be a thickness issue rather than a scratch issue. Ever notice most bondo failure (less than 1/4 in thick) comes in the form of little bubbles? When you dig it out- theres a rusty spot in/around it- this is moisture that was absorbed, and caused the rust, and then we see the end result. Had there been a barrier (epoxy) on the metal, moisture would not have mattered. This is how this was described to me, it makes sense to me and seems to match what I have seen. It might only apply to cars in my area, and maybe not in the NE or Seattle, or even arizona.
The filler manufacturers seem to all say to apply the filler to bare metal. I think they say since it would be hard for them to test adhesion to all of the different brands of primer. It's easy for them to test bare metal, so that's what they recommend. It also absolves them from any reponsibility for adhesion issues to primer since they can just say that you did not follow their recommendations. The primer manufacturers usually say that you can do it either way.
With that being said, I am with the majority of the responses - epoxy primer on bare metal with filler applied over the epoxy primer. After the filler work was done, I shot more epoxy primer over the filled areas before shooting high build primer. As dzahm stated, it's a sandwich. It will work either way - it's your decision.
'66 Emberglo Coupe - Restoration COMPLETE!
5.0 EFI conversion
TwEECer EFI tuning
Rod & Custom Motorsports IFS
TCP subframe connectors
Vintage Air Heat & AC
If you learn how to do your metalwork right you can significantly cut down on the need for fillers. Took me a bit to learn, but most of my leveling is done with some cuts with a file on the metal and a few coats of a filling primer. That is usually on both sides of the metal.
I used a lot of filler on my car, but it's very thin, as it's been skim coated. It was applied to metal that was scratched with 40 grit and it seemed to hold well. I used Evercoat brands, which applied nicely.
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