scuff to at least 400 grit and clean agressively. For old dirty parts I like to clean with laquer thinner first to get the heavy grim off. Then use a maroon pad (about 350 grit) and scuff with a water detergent solution until water is break free. Dry well and clean with wax and grease remover. If you care about adhesion, you will do a quick scuff again between dry and W&G steps, then primer application within 2 hours of scuff.
Many ways to skin that cat, but you must have etching (either chemical or mechanical), unless using chemical convertors and adhesion pomotors like alodine and it MUST be clean of all oils. While chemical etching is fine, I am not a fan of putting acid in my paint and therefore do not use self-etch primrs, though I would be fine using a chemical etch that is rinsed off before paint application, but you need to be d@#$ sure you get all the acid out of the nooks and crannies.
Larry
