Originally Posted by peregrines
For cellulose based thinners (the traditional head cements that smell like nail polish) you can use plain old lacquer thinner from a hardware store (I use it on Sally Hansens's Hard as Nails too).
Ditto's to Mark, I have a 1 Qt. can in the garage and keep a small bottle (4 oz.) on the bench.
Even if you use the correct reducer remember this, stir the reducer into the cement. Don't give into the urge to put some in, cap it, and then shake the cement. This results in a million tiny air bubbles some of which linger for days. These almost microscopic bubbles will create a very strange effect when the cement dries.