razor blade is probably the best idea for removing the positive sign. but you have to be extra careful sanding plastic!!! if you use anything coarser then 600 grit youll need to prime the part and then sand it again with 600 or higher wet paper. what happens is when you sand on the plastic it actually kind of creates these little furrs and when you paint over them it just looks really rough. so to get rid of those furrs youd have to use a primer filler over the sanded plastic and then when you sand the primer down it will smooth out all the roughness and furrs. the easiest way would to just be as exact as possible cutting off the sign with a razor blade and then just scuff the rest of the piece with some GREY scotch bright.