I had a terrible experience in class today. As I was working on my MBP the kid in front of me leans back (stretching, I assume) with un-capped pen in hand and puts a 1.5 inch pen mark on the top cover. I wiped it off quickly which got some of it but there is still a mark, it's not overly noticeable but I think you can also understand why I want to get it off. I tried wiping it off with Bar Keeper's Friend which had no effect positive or negative. Any other ideas?