To promote adhesion in spite of their oily surface the factory used a trick. The paint in the dip tanks was thinned with lacquer thinner which in fact dissolved some of the oils and at the same time speeded up the air drying process. This is extremely significant because that thinner imparts an initial high gloss to the paint but mellows to a nice semi-gloss in a few weeks. There were NO secret or magical flattening agents used.
Here is how to do it. With all components thoroughly sandblasted and clean, apply several coats of gloss black alkyd enamel directly on to the bare metal having reduced the paint only with lacquer thinner. If your painter has been well trained he will resist the concepts of no primer and lacquer thinner as a reducer but this is how it must be done.
This dried paint surface will be extremely durable and will last for years. It will bruise but it won't chip. Any area that fails will be because it wasn't clean enough! It will exactly duplicate the original factory appearance as well as the engineering spec.'s for the paint itself. It is by far the easiest finish to touch up, compound or repair.
Factory spec.'s will state that the paint on a frame be soft enough to allow the seating of mating surfaces, lockwashers to bite and all fasteners to torque properly. Powder coating, high build paint and other hard coatings will interfere with all the above.
I've mounted thousands of powder coated wheels in my trailer manufacturing business over the years. They MUST be retorqued several times and we still had them loosen on the road.