I use a water-resistant marker to label the CDs. So far, all of them work fine, but I read that if you write directly onto a CD, the writing will eventually "eat through" and ruin the CD.
Blank CDs say you should mark them with a soft pen marker only, so any of those should work.. nevertheless, I used to write on them with a biro and even with pencil (which is insane, yeah ) but nothing ever happened to them
lol, cds are very resistant! ive handled mine with wet hands, written all over them, dropped them, wiped them with tissues and paper towels, etc etc - you name it, done everything bad and they still work perfect. cds are resistant!
The data on the CD is contained on the underside of the label, so scratching the plastic on the CD is actually not as devistating as scratching the label. I've never seen marker "eat" through the label. I suppose you could just buy acid-free pens to be safe.