You don't really think I'm going to put this back in a game with those stains on it, do you?? And trust me, no matter how much you clean it, they won't come out.
Here's a little trick to make the PCB panel look better than new. DA sander with 180 grit paper, then 220 grit, then 320 grit then finally red Scotchbrite pad. This whole process will take roughly an hour but the final results are worth it.
BTW This one I got from Chris Hutchins.