you seriously need to use a higher grit sandpaper bro, and when your done use some rubbing compound (just dont get that shit inside your DS). i actually give you props on your 'mod' (matte black looks pretty good on electronics if done right, anybody with a black lian-li case can probably agree) but it seems you're really going about it the wrong way. not to mention that if your going to do a mod like this, go all the way, dont do some spots here and another spot there.
that said, gloss black looks spiffy as hell too (psp owners can probably agree with that). its all personal taste.