how much damage is acceptable?

[72CSTC5]

72 Longhorn BB, I am a certified Silver level PPG painter. I am not suggesting you use PPG products but I can give you some good advice if you want. The black that is on the parts you bought is EDP primer to prevent the part in rusting on part shelves and in your garage while you wait installment. You merely just have to sand those parts with 320 dry sandpaper and apply sealer and paint. In your case however where you are going to weld in parts, only grind the metal off in those areas. In the event let's say that you strip all of the paint off of your door that has some surface rust in those spots. I would apply naval jelly and scrub with steel wool until all surface rust is gone. Wash and rinse clean and dry asap! You may have flash rust from the water air and humidity. Sand that off with a red scrotch brite. Then you need to metal prep the metal with phosphate and water mix. The metal will look like you have a film on it almost like "bluing" a gun. Rough up the metal again for adhesion with lets say 180 grit sandpaper and blow off residue clean. Then apply a "metal etch primer" such as PPG 1971 that has "chromates" in the primer. That is the key word. Check to see what type of metal etch primer that you use has chromates in the primer. If you use this brand of product you only want to apply a very thin coat. You should be able to read a business card through the primer for example. You do not have to worry about covering all of the metal. While it is still "wet" you need to apply 2 to 3 good coats of epoxy primer over that. This is the best current method to date that companies have discovered. Do your bodywork over the epoxy primer if you can. This will give you the best corrosion protection possible. This is of course a perfect scenario, most of the time you break through the edges of the primer while doing bodywork. Just primer over your entire repaired area again. Good luck, if you have any questions you can PM me direct.