Old vart is correct.
If you have a hodge podge of crap to hold down, i.e. paint,old primer, some bondo an epoxy primer will hold it down the best...including some specs of rust etc. Its water resistant almost water proof. Its really just a bunch of glue. I like ppg DP40 and you can get it in colors but cheaper generics are good too. Epoxy primer is not rocket science.
A high build primer / surfacer is good too and can be used as it can be applied over old surfaces too but epoxy is a better base to work with as it holds down better. Urethane primer will not block water like an epoxy or hold down rust as well.
Most people like myself who do mess with classic cars use epoxy over everything i.e. old bare metal with paint etc., or sand blast / sand the entire car down than epoxy to seal the body. Bondo over that (yes you can bondo over DP40), or you can put the bondo on metal, epoxy prime, primer surfacer, block it out, and then a sealer coat that is tinted real close to the base coat so you can reduce the expensive paint needed for coverage, base coat and then a ton of clear and wetsand. You can skip the sealer if you tint the primer surfacer but depending on the brand of paint and their recommendations they may state they prefer the sealer coat to seal up the primer surfacer first. If doing metallics I say do a sealer coat.
New cars are different, most replacement panels are e-coated so you just clean, scuff,clean spray and go. Epoxy is not necessary as there is no junk to hold down.
The only time you can get into trouble is mixing too many different brand products. I would epoxy prime any brand and stick with a brand for primer surfacer / sealer and bc/cc.
I have been messing around with Matrix paints and they are cheap to learn with and are ok. My mexican low rider friends / painters swear by their top line clear coats (the 52 or 42, whatever is the more expensive one) that are ppg 2021 copies but I have not tried them. Pretty cheap. They toss them over house of color and have no problems.
Sean
