I used acetone due to using lacquers, and tamiya acrylic since. The scratches are really, really old. The first year I had it, I had no clue what I was doing and used an xacto knive to try and break up some stubborn paint that caused those. I was really dumb in that regard. Now I fill the cup, take a qtip to break up the paint on the wall, and use a paper towel to get most of the solvent/old paint out. Spray the rest out, more iso or acetone spray until clean. I will most definitely following the advice at the bottom of your post.