Paint and body work...

[metalmate]

Great replies to this post! I know from experience how NOT to properly apply bondo type fillers. Fillers applied over bare metal or lacquer based primer and/or sealers will not last long. The repaired area will rust and the bondo/filler may swell up and/or the paint will blister from outgassing of solvent/hardeners.

Self etching primers are acidic on the pH scale all other primers and sealers, as well as most fillers are on the alkaline side.

Seal the bare metal with self etching primer or epoxy based primer, let it dry then apply the filler. What the others said about letting fillers dry/cure completely is VERY important for long term success in a repair.

Yes it's difficult to wait days or weeks for filler to fully cure prior to getting on with the paint job but, it's important. You can use an infrared curing lamp to speed things up. This sounds silly but, if you can smell the filler it ain't fully cured.

[PetesPonies]

ok, some facts for you. Body filler is polyester plastic. There are different amounts of talc added to the fillers, to create thickers or more creamy fillers. Each has it's own usage. Filler applied over clean bare metal is NO problem at all. Dents etc, where there is no rust can have filler applied directly. It is and has been done this ways for 50 years with NO problems. The problem occurs when the metal is not perfect. Over welded joints, or areas of metal deteriation of some sort, an epoxy primer applied first will make for a watertight seal. I spend a lot of time making sure my welds are not porous; no small opening left. but it is very easy to miss some. Epoxy primer will seal these up and create a good substrate for filler application. I belioeve epoxy primer is the best material to put right over bare metal. Many will want to use etch primer. Etch has an acid in it to bite into the metal. However, etch is not a waterproof material. If epoxy is applied over the etch, then that is better. however, epoxy has great adhesion properties itselff. I prefer to etch the metal myself with phosphoric acid, apply the epoxy and know that I have a good surface, one that should last. Many will apply the epoxy without etching first. Epoxy is great stuff; remember using epoxy glues??