Auto Restoration 101

Body filler is probably the most misused product there is when it comes to body repair. Got some rust? Just fill it up with bondo right? WRONG! There is a proper way to mix the filler, apply it and sand it. The sanding is a lot of work and takes practice to know when you have it straight.

1) You have to apply it over CLEAN PROPERLY PREPARED METAL.
2) Yes, you can apply it over properly prepared epoxy primer. If you apply it over epoxy primer make sure the primer is fully DRY and also SANDED.
3) You shoudn’t mix it on cardboard because it soaks up the resins. Use a plastic board made for mixing body filler or a clean piece of metal.
4) Mix it properly, using too much hardener can cause problems later on. Follow the manufacturers directions.
5) It should never be over a 1/4″ thick. I have seen it as thick as 2″ before!
6) Apply it in thin layers, when applying the first coat press firmly to force it into the metal.
7) Never apply filler over any type of etching primer.
You never ever apply it over rust, cracks in the metal or use it to fill holes. Moisture will eventually get in underneath the filler and ruin all of your hard work.
9) It is critical that after sanding the filler and before applying another layer that the filler be cleaned off with dry compressed air. You will be suprised at how much dust will come off exposing small pinholes and imperfections.
10) Rough up the unsanded areas before applying additional coats.
11) If you apply it over a brazed weld you will definitely need to cover the brass with epoxy primer. Body filler does not like to stick to brass for very long. It will eventually bubble.
12) Start with 36 grit, then 80 grit and finish with 180 grit. I mainly start with 80 grit and finish with 180.
13) Bonus tip! Wear a dust mask when you sand.

There is an excellent article on the subject of body filler written by Reed Overson called Using Body Filler. I agree with everything that he wrote in this tech article except that I have and still do use body filler over epoxy primer and have very good results with it. I also use Zero-Rust instead of POR-15. Print this article off and keep it for reference.