Depends on the type of plastic it is. "Plastic" isn't a specific material: it's very, VERY general term for A LOT of very different materials. If it's something like ABS or polystyrene, then a primer with a "hot" solvent base (like lacquer) is all you need to get best adhesion. If it's something like melamine or bakelite, primer solvent "heat" won't make a difference, but with proper cleaning, most paints will stick well anyway. If it's something vinyl based, paints will stick, but oil-based ones (enamels, some lacquers) will never cure properly, leaving a permanently tacky finish. If it's a non-polar plastic like polypropylene or polyethylene or delrin, then even scuffing the plastic will really only half-work at best.
Check the bottom or inside of the part to see if there's a recycling code. That'll narrow it down to where more specific/reliable suggestions can be made.