I've been researching paint and body work for a while, and I've got to say I'm frustrated. It seems as though everyone does it differently, and it's hard to find a complete list of steps. I went to a local painting supply shop, and I got this advice on steps for prep/painting. I'd really appreciate it if any experts here can step up and point out/explain anything that isn't correct. I'll come back and fix my post to reflect the advice so it can serve as a reference.
1. Clean the part with grease and wax remover. The local shop gave me "Prep-All" made by Klean Strip. You wipe it on and keep the area wet for 4-6 minutes, then wipe off with a dry cloth. I did this step several times, until very little was removed. The object is to remove all contaminants, because they will affect the paint job. Oil, wax etc won't come off with the paint, it will be ground into the metal, causing problems later. Work small areas, and no heat sources, pilot lights or flames nearby!
2. Remove the paint and rust. 40-80 grit sandpaper on a grinder will work for this.
3. Use metal cleaner/etcher. The local shop gave me PPG Chemfil DX579. It's phosphoric acid (like naval jelly, only a liquid). You dilute it with water, and then spray it on from an acid-resistant bottle. You wait for a color change in the metal, then rinse it off and dry with clean, lint-free cloths. I've always been bothered that the metal flash rusts almost instantly at this point, but the paint does seem to stick fine and I haven't had issues with rust coming later.
4. Tack rag.
5. Body filler. I've been told to use a little at a time (golf ball size), let it cure slightly and then use a cheese grater - style file to get it close. Once it's dry it's hard to work with. When the filler dries, use 40 and then 80 grit to flatten it. Use lacquer thinner for cleaning filler off tools. Tack rag.
6. Two part glazing putty to fill imperfections from the body filler. Tack rag after sanding with 120 grit.
7. Prime. Start with several coats, then lay down one coat of a contrasting color. When it's dry, use 600 grit to sand. The top coat is a guide coat to show low areas on the part.
8. The local shop also gave me Evercoat Metal Glaze pn 100416 to use for larger areas if necessary. This is used to fill larger, shallow low areas shown by the primer guide coat. According to them, it's similar to body filler, but easier to work with when it's dry. Also it is used over the primer, whereas the body filler is on the bare metal.
9. Repeat 7 and 8 until satisfied.
10. Paint.
This sequence is not exactly what a pro would do. I don't have a compressor and spray gun (yet). My current project I'm taking up to step 9 and then having someone else do the base/clear.