A: Quinn, your problem with paint adhesion is likely due to some kind of dirt or residue on the piece rather than the brand of paint you used.
First of all, I recommend that you strip the metal castings clean with paint stripper (use a brand that works on metal) or by boiling the castings for about a half-hour in a pot of water in which you have added a half-cup of powdered laundry detergent. Common sense says don’t use this pot for food, and check with your spouse first!
Then, rinse the castings thoroughly under running water to remove all traces of the stripper or detergent.
Handle the castings carefully to avoid getting fingerprint oils on the surfaces to be painted. Allow them to air dry.
Whether you use a primer is up to you. Lionel didn’t and I usually don’t, but it doesn’t hurt. If you use one, make sure it is of the same brand and type as the paint you intend to use.
Primed surfaces often take paint better than unprimed ones, and some primers can be sanded before the final coat of paint, which gives you a smoother final finish.
Many train restorers use off-the-shelf cans of spray enamel with good results. For example, I have found that Krylon semi-flat black does a good job matching the color of postwar steam engines.
I did an article on painting in the March 2001 issue of CTT, which you might find useful.