The clear coat applied to auto finishes is essentially the same material as the paint -- except for the ingredients that add color. It's clear paint. Wash the surface to free it of anything that might be stuck to it, including water soluble things (dirt, oils and such). Then remove stuff you need to use solvent on (like road tar). With the surface clean and free of anything but the original finish, sand it (or just scuff it in all areas to be painted), mask it off, wipe it down, and apply your new finish.
Preparation is the key to a good finished product, and the best paint job starts with the best preparation. No good painter will apply a coating over an improperly prepared surface. Said another way, paint jobs are made or broken in the prep work.