I came across one that looked as if it had a microscopic crumb on it, like a cookie crumb. So I thought I could simply flick it off with my finger.

Bad idea. To make a long story short, the more I rubbed it, the more the stain [[by now a scuff mark) became and I have not been able to clean it satisfactorily. I have since tried to play the CD, which was fine before, but now it skips and just stops altogether.

So have I ruined a perfectly good CD or is there hope that I can restore it to playability?