I painted some spoons with acrylic airbrush paint and topcoated with Dick Nite Fishermun’s Lurecoat, which is a moisture cured urethane. The Dick Nite seemed to penetrate the paint and adhere directly to the metal, forming a very durable finish. No primer was needed. Dick Nite is a spoon manufacturer who also sells paint and topcoats. Mark above mentioned an important detail - your primer, paint, and topcoat are a related system. Not all paints work with all primers and not all urethanes work with all primers. I tried an aerosol auto primer on some spoons and hated the esthetic result and the strong residual smell. Bottom line, you may have to experiment to get a finish you want.