I originally thought that CDs were plastic discs with foil that contained information. I was wrong! Only recordable discs are like this.
(also see Wikipedia)
Lucky for me, because I've seen a few CD's that had missing foil. Which brings me to the next section.
Repairing Foil Scratches
First, a quote from another website (lifehacker? gizmodo? something like that).
This works for removed foil and minor scratches - as you can see, the pits are supposed to be protected by acrylic (which would be gone if the foil is gone).Scratches on the label side that let light through when you hold the disk up to the sun are fixed with a dab of White Out on the label side. These scratches removed the aluminum reflective layer and can cause the choppy or skipping sounds. I always hold the library CD up to check for this problem.
Although I might consider some sort of liquid foil.
Another quote, from Metafilter:
The first thing that comes to mind here is gold leaf, which is extremely thin, very sticky, and should be excellent for the lasers that cd drives use. And, rather than gluing, one might consider the gold leaf underneath an adhesive CD label.Repair the top side where the label is,; gluing foil over scratches.
CAUTION: A CRACKED DISC CAN SHATTER AND DAMAGE YOU AND YOUR DRIVE
Scotch tape has been recommended (along the crack, and the same amount on the opposite side to balance it out). More likely, a thin bead of runny super glue (completely dried) would work.
Buffing out Scratches
Rather than "waxing", buffing is the method for removing scratches. One thing to consider is that the light beam is wider than the data pits, so a mirror finish is not needed. Toothpaste is common, as is Brasso(Tutorial) polishing compound (and other metal polishing substances).
There is also an extensive guide here on cleaning and polishing a disc, as well as disclaimers.
(from a CDFreaks Post)
Another forum recommended a Plexiglass Repair Kit (link) or Plastic Polish (linkAnother NEAT way I use to fix BADLY scratched CDs is to clean them as best you can. Then I have a portable CD player that I have just the motor working in on a little toggle switch on the side. I have the rest of the guts taken out of it. I put the CD on the spindle upside down, (it has to be the type where the CD will stay on the spindle with the lid open) and I put a few drops of the runniest super glue I can find around the middle of the disc. Then I close the lid and turn on the motor. It slings a nice even coat of super glue across the surface of the disc filling in all the scratches, and slinging the excess off the edge. I turn it off after about 20 seconds, and give it like 20 minutes to dry before I open the lid. Then I take out a disc that looks BRAND SPANKING NEW, you'd never know it was scratched!! It only works IF the reflective layer is OK, and I've had LIMITED success doing DVDs like this. BEWARE, some super glue clouds the discs, so TEST IT FIRST. Take a CD-R and put some on it, and see if it turns cloudy after a few days. If it doesn't, it will work. I had this really cheap super glue that clouded after a couple months. I noticed it happening, so I copied all the victim discs I had done that to before they got completely hosed. I find the closest to the pure cyano???? it is works the best (not sure what the name of the chemical is). So, there you have it, my secrets for fixing screwed up CDs.
A third (link)says that for really deep scratches, an "optically transparent material" combined with a spindoctor type product (maybe try after the superglue technique above) works pretty well.
You can also have your local movie shop (or music shop) use their machine to resurface it - usually costs between $2-$5, or buy one yourself ($200-$5000).
isoB uster and Exact audio copy are recommended - I'll add more in a bit