Can I varnish Painted Plastic?

I recently painted a plastic table and want to protect it from scratches. Can I use multiple layers of varnish like i would on a wooden surface, or is there a better way to seal ant protect the finish.Can I varnish Painted Plastic?
you dont use varnish over painted surfaces you use it over stainCan I varnish Painted Plastic?
You could use a spray polyurethane. You don't want to do a brushed on finish. Polyurethane, varnish and shellac are too thick and will yellow the color you just painted. Check the hardware store for a clear sealant made for outdoors. This is generally made for a variety of surfaces.