Quoted from vid1900:Clean with Naphtha after sanding. Don't get oils from your hand on the clean surface.
I've been slowly reading through this thread from the beginning, and saw this caution. I used to work professionally in animation as a cel painter (inking drawings onto acetate and then painting them). We would use cotton gloves to prevent transfer of oils to the surface of the art. The gloves are cheap and can be washed over and over again. We cut the tips off of some of the fingers and thumb so that we could maintain a good grip on the tools we were using. I haven't done any playfield restoration yet, so I'm not sure if they would be as useful for this work, but thought you might find them helpful.
http://www.cartooncolor.com/1-doz-pairs-cotton-inking-gloves/