Fixing scratched CDs with industrial abrasives or whatever

[tgies]

That polishing-a-scratched-CD¹-with-Brasso trick works. I just went from a CD which Would Not read to a CD with minor but recoverable read errors which passes an AccurateRip² check upon ripping. Pretty rad! I've been trying to get that damn CD to work since forever. I suspect that it is still going to skip in some CD players, but that doesn't matter now, since I have the bit-exact samples off the disc now and I can make a new one.

The trick to CDs is that the data are physically stored directly beneath the reflective foil layer. Few people seem to realize that scratching the top of a CD is seriouser business than scratching the bottom. Even the tiniest hole in the foil ruins the whole works — there is nothing off of which the laser beam can reflect, and the drive will pretty much sit at that spot forever, trying to adjust the optics to find the correct focal point. If that foil is still there, it is usually possible to polish away any beat-up plastic on the shiny side which is stopping your CD from reading properly. Brasso metal polish contains extremely fine particulate matter and a moderately strong solvent which together work to remove some of the plastic from the shiny side of the CD, taking with it any scratches in the plastic.

To do this, you want some Brasso metal polish (the kind which is just the white cream crap in a bottle or whatever, not the cloth stuff that's already soaked in it) and some kind of soft, lint-free cloth — I used a really old Army-issue undershirt, but there are probably better things to use, such as an optics cleaning cloth. Squirt a bunch of Brasso on there and rub it radially³ (inwards/outwards in a straight line between the center and the edge) for a while. Add a little more Brasso when you get it totally dry, because dry Brasso powder crap is going to unevenly abrade the surface and cause problems.

Go at this for several minutes and rinse it all off thoroughly. You're now probably looking at a CD with a whole whole lot of really fine radial scratches, perhaps looking more like a sort of uniform haze than actual individual scratches. This is less problematic than you might expect. If you've polished the disc enough, you should find that your original large scratches have either disappeared or become less visible.

Try reading the CD now and see if it's not better. I found with my test CD that there was a point where it got worse, and then, after several more minutes of the same polishing, a point where it got much better. Keep at it if it doesn't work the first time — the disc I repaired was pretty beat up, and I must have spent about twenty minutes on it.

Finally, and somewhat unrelatedly, try reading unreadable CDs in a DVD drive. These usually have more sophisticated optics and a more powerful laser, and are more forgiving of scratches.

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¹ Whenever I write "CD", pretend I wrote "optical disc".
² AccurateRip is a service which enables one to verify that the audio data has been correctly retrieved from an audio CD by comparing a hash of such data to that generated for the same track of the same CD by other AccurateRip users. This naturally only works if another AccurateRip user (preferably two or more others) has already ripped the same CD, but I find that most of my CDs are identifiable with a confidence of at least 3.
³ The data stored on a CD are recorded in a spiral pattern outward from the center. While scratches perpendicular to the direction of this spiral will only affect a few bytes and can be corrected for (data are stored redundantly on a CD), scratches approximately parallel to it are likely to affect a greater number of sequential bytes on the disc. Polishing in a radial pattern avoids the introduction of such scratches.

Comments

[bdu.livejournal.com]

Back when I was making daily software release builds, I was using CDRWs to burn the installers to save on the waste production. I noticed that the life of these CDRWs wasn't nearly what it ought to be, and each one would only last me a couple days before failing verification on a burn.

Upon closer inspection, they would each develop long radial (to the center hole) scratches on the bottom of the CD, obviously the cause of the problem. I cleaned out my burner, the primary reader drive they were used in, but nothing made the problem go away, so I resigned myself to using a new disc every couple of days.

But one day I happened to notice that a certain coworker, whenever he was bored, would wander by, look at the surface of one of my CDRWs, notice a few flecks of dust, take a corner of his shirt to the bottom of the CDRW, and wipe the disc radially to remove the dust.

I had to carefully explain to him that a little dust was unlikely to cause a problem, that he should leave my discs the hell alone, and that if he ever finds himself needing to clean a disc with his shirt, he should wipe from the the inner hole to the outside edge only, and never in radial patterns around the disc.

[tgies.livejournal.com]

Doesn't "radial" mean radiating outwards from the center like the spokes on a wheel?

[bdu.livejournal.com]

Uhh, yeah, can't brain today, have teh dumb. s/radial/circumferential/g

[tgies.livejournal.com]

I actually wasn't sure if I had the absolute wrong word myself for a moment :G

ps we all know CD-RWs don't actually work anyway