Mineral spirits are perfectly safe for key buttons. It's critical to make sure the solvent you're using really IS true mineral spirits, though.Other solvents, such as those billed as "laquer thinner" are NOT safe for plastics. That includes acetone, xylene, and any solvents containing them. I have heard that Selectric III keys are resistat to laquer thinner, but I still wouldn't use it even on a III. Laquer thinner may be applied VERY carefully after the full mineral-spirits bath using a syringe or small squeeze bottle for specific metal pivot points such as interposer pawls that tend to get extra-stuck from dried-out lubricants.