You can sand the doll first, but it's not necessary. You do want to give your doll a thorough scrubbing with a magic eraser though, especially BBB/RS dolls, to make sure you remove all the mold release residue. So long as you don't seal the doll first, the dye will take directly to the resin, and shouldn't rub or fade much at all, except inside the joints, where there is friction. I've dyed 3 dolls using RIT dye, and fantasy parts for two more, and have never had any problems with chipping or fading. However, once dyed, you won't be able to scrub your doll to remove stains without removing the dye. I have a project thread
here for a full dye and paint job. I've got some basics on the process I followed in the first post, if you're curious.