So I'm grinding someone else's glass, so it has to be perfect.
It's a curved surface, and it's black, which shows imperfections oh so we'll
I thought I got to the Polish, but it turns out there are several areas that either need more cerium/ felt love, or maybe have to go back a step.
I've been using silver sharpy to mark these areas.

My question is, is there anything I can use to mark trouble spots that won't wash away as easy? Maybe something that only comes off with soap or alcohol?

It'd be a time saver not to have to think I'm done, rinse the glass, dry the glass, and search all over again looking for those spots to see if I got'm; find'm and re-mark, repeat, rinse, etc.