Oh WOW! That would definitely add a lot more challenge to it for sure. Have you ever made a sub do something like that? I'm assuming in that case, the best, and safest method would be a lot of up and down motions, sort of dropping the head of his dick on the sandpaper to blot the paint down, then lift up, move slightly up or down, then drop it down again to blot another circle of paint, and repeat to very slooooowly form the letters. Trying to make it go faster by dragging his dick on the sandpaper I'm sure would prove to be a major regret later. Using this method would be safer, but it would probably also feel like he was doing literally hundreds of mini pushups in the process.