It is perfectly possible to paint over your car’s existing finish - however if you’re painting over a clear coat, it’s advisable to sand it down and use a primer first, to ensure the new paint will bond to the surface. This will help prevent peeling, and give you a better finish. If your car is old, or you’re not sure whether it might have rust or other damage, you should consider stripping it back to bare metal before re-painting it. By doing so, you’ll be able to discover and treat any underlying problems, before you cover them in new paint.