I used spray-on bedliner, can't remember the exact name. I bought it at advance auto parts, but I'm sure they sell it at any other auto parts store. I also bought a metal degreaser/pre-paint cleaner which I used before I sanded, so as not to dig the grime into the paint, as well as after I sanded. Taking extra steps in preparation makes the paint adhere better and last longer.
good luck!