the first part is you have to remove all the old paint, this can be done with hard work and the correct type of paint strippers, or it can be done by "sand" blasting by someone who knows what they are doing, it needs to be done by a pro who has done it lots before, the metal is very thin and the heat generated can distort it, or at worst, eat right through it, next comes the actual paint, best advice is to go to a paint specialist and ask questions, there will be a dozen brands and probably all are OK, but I tend to use the one brand of etch primer, undercoats and topcoats, and follow the instructions carefully.