I've had luck wetting the media inside the floppy with isopropyl alcohol and either spinning it by hand for a bit or running it in the drive, of course it doesn't always work but it could be worth a shot. Sometimes if you look a the disk through the window you can see where the problem is and clean it with an alcohol-soaked cotton swab.
I'm kind of shocked you guys have tried using alcohol on the actual magnetic media itself. It's usually recommended for cleaning the HEADS of disk drives, audio tape players, VCRs etc., but NOT the magnetic media. The magnetic media contains a powdered oxide which could potentially be wiped away or damaged by touching it with fluid. Even if you did dare touch it, you'd need pure alcohol....most of the stuff you buy from the pharmacy is only about 70% alcohol (the other 30% being WATER).
Most of the errors on a disk are caused by drop outs (flaking off of the aging magnetic particles from the mylar backing of the disk) or loss of magnetic integrity/demagnetization. I can't see how any amount of cleaning could remedy a dropout.
However, you guys have obviously had some success...so maybe all that I've come to understand about magnetic media over these past several decades needs to be re-examined. Can anyone explain how cleaning a disk surface would actually work (I suppose if there was actually enough dirt on there to cause the head not to be able to get near the disk surface to read the data)?
