Invest in some wax and grease remover. Autobody grade. If it's really bad, use some Dawn dish detergent, about 1/10 ratio. Wash the piss out of it. Rinse. Dry. Then use your wax and grease remover. Then start sanding. Otherwise, your're going to gum up the paper with tar/grease/contaminants, and it's like dragging a rag full of rocks and trash across your paint. Won't be even at all.
400 is a good place to start. Be careful around the edges, they wear straight through to metal very quickly. And for the love of god, USE A BLOCK. Otherwise it's gonna look like the ocean. Anywhere you hit metal you'll have to use an epoxy, so just sand enough to get the whole thing a consistent dull tone.