Do it on plywood in your back yard first. Going from a sketchbook to a wall is totally different.
Then paint over the board. No evidence. A friend of mine's buddy got fucked over in MT because cops got a warrent and found his book, it matched some pieces he had put up in Missoula and Bozeman
if you're asking these questions, you probably shouldn't be considering painting yet. stick to tags and your blackbook for a while, you'll be happy when you look around and don't see awful throwies you did a few months ago.