Find a paint dealer who supplies collision shops. There are a couple in every town that has more than one auto body repair shop. Take a part or two and ask the counterman what you need to paint them. There are prep solvents and adhesion promoters for plastic parts. The stuff in Rustoleum or Krylon is not the some and probably not quite as good.
Most of the plastic bodywork on classic k bikes is polyester bulk molding compound and can be painted with almost any good automotive paint system. The only injection molded flexible plastic parts I can think of are the mud guards, mirrors, tragkorbs, fairing inner panels and belly pans
About primer. If you used a high build primer you should expect to wet sand with 600 grit paper. That stuff is designed to be sanded for a perfect surface. Really, you should always plan on a light wet sanding with 600 before any color coat regardless of what it;s going on.