crashtech
Combo Man & Mod
Filler manufacturers will recommend that filler be used on clean, rough sanded bare metal. Most shops still do this, and it's pretty much the way most of us were taught. There's reasons for this, mainly that the filler makers can't control the quality of the coating on which the filler would be applied, so the safest thing for them is to specify bare metal.Jeez, I hate to change course in this thread but crash I have got to ask about a comment you made, as it is something I thought differently about. Your comment being, "and never apply filler on bare metal". Keep in mind, I don't know much and just asking to clarify, as I have read that filler could go on bare metal or over epoxy. In fact, it seems I read that the epoxy would be roughed with 80 grit prior to application of filler. Would you enlighten me here on this? I know you know your stuff and I like to know the why's to things.
BUT, filler is porous and does not inhibit corrosion, so it's not a great coating for metal. If a filler on metal job gets even a tiny, unnoticed chip, it can absorb moisture into the filler, and then the panel will begin to rust underneath the filler. This is a very common failure mode, the filler just turns into a dirty (salty, too, in some part of the country) wet sponge up against the metal. If epoxy is properly applied first, it creates a corrosion resistant barrier that won't allow that dirty wet sponge to touch the metal.