This article is from the CD-Recordable FAQ, by Andy McFadden (firstname.lastname@example.org) with numerous contributions by others.
If you scratched the top (label) side of a CD-R, and it no longer works,
your disc is toast. (If you scratched it, and it still works, copy the
data off while you still can.)
If you scratched the bottom side, then all you've done is etch the
polycarbonate (plastic), and it can be repaired like any other CD. A common
misconception is that the data is on the bottom, but if you examine
it carefully you will see that the data is beneath the label. The
laser reads the data through the polycarbonate layer, and if the layer
is scratched the laser will refract onto the wrong part of the disc.
For small or radial scratches, the error correction in the CD format will
allow the disc to continue working, but if there's too much disruption
you will get audible glitches or CD-ROM driver errors.
If the disc works some of the time, you can "repair" it by copying it onto
a new CD-R disc. If the disc is always unreadable, or is copy protected,
you will need to repair the disc itself.
One product that may be useful is Wipe Out! (http://www.cdrepair.com/), a
chemical abrasive that allows you to reduce scratches. Another is
Discwasher from http://www.discwasher.com/.
The Repair FAQ at http://www.repairfaq.org/ has a section on repairing
scratched CDs. Find the "Compact Disc Players and CDROM Drives"
section, and skip down to 4.10 and 4.11. Details on using common
household chemicals (e.g. toothpaste) to fill scratches can be found at
Some people have suggested using plastic polishes or "fine cut" paint
polishes sold for removing fine scratches on automobiles. These fill in
the scratches and create a more optically consistent surface. Fine metal
polishes may also work, and some people claim that plain old white
toothpaste does the trick. There is some chance that the filler material
will fall out over time, rendering the disc unreadable once again, and
possibly gunking up your CD-ROM drive along the way. If you want to fill
in the scratches, you should make a copy of the contents to a new disc as
soon as possible, and stop using the original.