How to remove solder mask?

Google "fiberglass brush pencil", once you try that you won't want anything else.

Usually a little wet/dry abrasive paper does the trick, use it dry first then damp it a little.

You may get the same results with fine sandpaper, but I think wet/dry is better as it has a finer grain, which is less likely to damage copper traces (especially thin or shallow ones).

That's how I did it when I got a faulty batch which did not have enough mask clearance.

I've used an electric eraser equipped with an eraser stick that was the firm pink colored one that had some bite to it. This would be similar to the eraser on a #2 pencil. These have a certain amount of fine grit in them that will remove the solder mask and shine up the underlying copper. (Of course if you grind away at it long enough it would also remove the copper too).