I was told not to use it on the inside, unless its going to rinsed really well. I did a tank a while back and haven't set it up yet. I did use a orbital car buffer with it, with a high quality pad.
Some of the scratches are still vivible, but I put enough elbow grease in it. When I rinse it out and clean it before using it, I'm going to see how visible the scratches are with water and lighting. My friend did the same to his tank, but it was in worse shape, most scratches disappeared with the water, but I think it'll depend on the lighting on it too.