'media blasting' is best. sand will work, but heats the metal up more and may warp it. godd old fashioned sand paper and elbow grease it tried and true method.

any large amount of blasting should be done with teh car apart - you will get blast media everywhere, and that will create problems.

chemical stripping you can try...toxic hazardous and you need to know what you are stipping to pick the right chemical. and you MUST get all the chem off or the new paint won't stick.