I assume you are not trying to remove stickyor dried out handlebar tape gunk, rught???. If it is color you are trying to get off the handlebar, it might be because it's colr anodized, not painted. Same thing happened to me when I was cleaning and stripping off paint from the head of my bike's aluminum seapost. Everything came off, except for the two clamping pieces at the top of the post, kept putting the stripper on it, but no cigar. I looked at the pieces more carefully and it turns out they were anodized black and not painted.... so now I have a two-toned seatpost clamp...which actually turned out looking real nice!

Chombi
84 PeugeotPSV
85(?) Vitus Carbone Plus 7