good info in this thread, let me add something for anyone who does not want rust coming through...
DO NOT USE RUSTOLEUM AUTO PRIMER!!!!!!!!!!!!!!!!!!!!!!!!!!
I used it on the bare metal on my dad's 84 before paint and freakin 3-4 months or so later rust came out. I figured you know what the heck it's not bad on price like 3-5 bucks a can, it's spray can so it saves on my mixing and spraying plus we usually buy the cheap stuff anyways, and I was doing one section at a time. So I didn't want to waste primer by mixing and not using it for a day or so since I'd only need so much to do said panel. Plus it's rustoluem aint they supposed to try and stop rust or keep it from coming on? It freakin says rust preventative, yea right my right butt cheek it's rust preventative, maybe if you don't use it, it would be.
I do know the self etching primer mixed in spray cans from the body shop supply work and I trust them over bare metal, they say it can cause old paint to peel or something if it's sprayed over it or cause body work to lift but I aint sure. I used it on my monte carlo on some bare metal edges and had paint/old primer on them and it never flaked or peeled. I also did the whole trunk which was spider cracked badly and had rust under it, I didn't even get all the discoloration out of the metal still had black and rust looking spots to it, sprayed it with the etching primer, painted it, never had a problem since. It's been over 4 years now and still is perfect.
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