For anyone painting signs or murals where there is a lot of sketching and erasing, I thought I'd share this tip. Sharpie shows through. Yep, I cleaned up all my pencil lines and of course the erase makes it's own subtle marks, but a fresh coat of white paint over all my painstaking work (it does give one a moment's pause) and then **poof** the background is all fresh again and the line you wanted to keep at the only ones showing.
Thought I'd share in case you didn't know this little hack.