Taking your time and properly preparing the surface is the first step in achieving success. You cannot get high quality results without high quality preparation. Unlike house paint auto refinish paint hides nothing. If anything, the high gloss clear coat system magnifies imperfections.
Before sanding, affected surfaces should be washed with mild soap and water. After drying, clean with some form of wax, oil and grease remover, often know as Prep-Sol. Prep-Sol removes road tar too. The best procedure is to use a clean lint free wipe to apply the Prep-Sol wet to approximately one panel at a time. Allow the liquid to work dissolving wax and contamination for a few minutes. Then wipe off dry using a fresh, clean wipe.
If the Prep-Sol is allowed to dry on the surface before wiping dry the contamination will remain on the surface and the process needs to be repeated. Contaminated wipes are not to be used over again in paint processes.