depends on how far you wanna sand down if you wanna sand it a fair bit down use a fairly corse wet and dry like a 120 or something then sand it smooth with a 4-800 paper wet and dry (when it is wet it works better)
then get spray putty to remove any inperfections in the sanding or old paint work whatever there is you can see then sand that with 4-800 grit wet then it should come up pretty smooth that is hw i do it whenver i paint parts of my car and it comes up well