The alcohol that I use is the 91% strength that is used for antiseptic wound cleaning, available from most drug stores. The common variety rubbing alcohol has a concentration of, I think, 70%. That may not be strong enough for a thorough cleaning. I wrap the neck below he strings with something like Saran wrap, as somebody else has mentioned, and then dampen a 2"x 2" white cotton bore patch, available from gun shops, and pinch the string with the alcohol wrap and then scrub the length of the string until clean. The white cotton patch has a slightly coarse weave which is good for scrubbing the string and the white cotton will embed and easily show any debris or chemical residue picked up from the string. The alcohol quickly evaporates with no residue of its own and leaves my D'Addario Chrome flats smooth, shiny and clean.