Note that epoxy primer seems to be a great way to prevent rust on bare metal, something regular primer does not do (and I can confirm that).
Mostly an original quote, but with a little reformatting to make it easier to read.
From: Radkins from the Home Shop Machinist forum, here.
Epoxy primer is great stuff and the first thing I do to any body panel or repair is to spray it with epoxy primer, the stuff is expensive but it can't be beat for corrosion protection!
A couple of tips for using this type of primer:
- It needs a rather rough surface to adhere properly. I scuff the bare metal with 180 grit paper and then apply two coats allowing it to become dry to touch before applying the second coat. If a finish is not applied within a day or so, this varies with brands, it will need to be scuffed with sandpaper and another coat applied before any topcoating is applied including conventional primers.
- If left for any length of time then it should be topcoated with another primer such as lacquer or one of the polyester filler primers. No scuffing needed if coated within a couple of hours. If left for weeks, never mind years, it becomes EXTREMELY hard making it very difficult to sand and causes difficulty in getting topcoats to adhere. If something like even simple lacquer primer is applied around two hours after the original application it will adhere quite well and is then easily prepped for painting even after being left for long periods of time, old uncoated epoxy primer however is a absolute nightmare to properly prep for finish painting!
Oh, and he follows up with this: (something else I learned today).
NO, DO NOT SANDBLAST BODY SHEETMETAL!!!!! A sandblaster will warp most sheetmetal body parts beyond any hope of repair! I don't mean "Aw s#!* now I am going to have to fix that", I mean toss it in the scrap and buy new parts kind of ruined. Some parts that are stiff and well supported like door posts and some firewalls may be blasted ok but fenders, hoods, trunk lids, etc will be instantly distorted way beyond any repair. Those sand particles act like millions of tiny peening hammers and will distort the surface of the metal due to the uneven stretching (and no, heat has absolutely nothing to do with it despite the common misconception so keeping the parts cool is futile!). While it can and has been done successfully the blast operator has to REALLY know his stuff and even then it's highly risky.