I'm assuming by "sanding down" you mean old paint removal. If that is the case, 36 grit on a dual orbital sander will make quick work of that old paint. Use etch primer on the bare metal and then use a 2k high build primer on top of that. The 2K primer will fill in the sand marks left by the coarse grit. Then apply a guide coat on the 2k primer and sand with something like 120 or 220 until the guide coat is gone. I suppose it would be helpful if you give us some kind of idea what your current auto body skills are. If you are starting from scratch, I suggest you visit some autobody websites such as autobodypro.com and autobodystore.com. Those websites have a ton of autobody information.