Hi guys. I have a previous version MacBook Pro, and on the keyboard after I cleaned the whole case and screen for the first time in about 3 months, I noticed that the keyboard, specifically on the B and D keys, there are little grey stains. I tried simply rubbing them off with no success, and then tried to scrape them off with a fingernail, with no success. It's not a big deal, just a little annoying. Any tips?