Naphtha dries (evaporates) much faster than mineral spirits (paint thinner). This leads many to believe that thinning polyurethane with naphtha will make it dry faster. This is not entirely the case.
Like all varnishes, oil-based polyurethane dries in two steps. The first is evaporation of the thinner. The second, and much longer step, is by the crosslinking brought about by the introduction of oxygen from the air.
When you apply polyurethane, you notice that it stays wet on the surface for a short time as the thinner evaporates. Then the finish goes into a tacky or sticky stage for an hour or longer. This is the length of time it takes for the oxygen-induced crosslinking to occur. Adding a faster-evaporating thinner doesn’t speed this crosslinking.
What the naphtha (or paint thinner) does do, however, is thin the finish, so you’re brushing or spraying less solids. After the thinner evaporates, the thickness of the finish will be less, so there is less to oxidize and the finish will dry a little faster. The problem, though, as you have already guessed, is that you will have to apply more coats to get the same total thickness, so the total drying time may actually be longer.
