Can you put oil base poly over water base poly?

Both water-based and oil-based polyurethane will be suitable for a floor finish, but your choice may depend on a few different factors: Water-based poly dries faster, leaves a crystal clear finish, emits lower levels of VOC's (volatile organic compounds = fumes), and requires only soap and water for cleanup. Oil-based poly takes longer to cure, gives a warm amber glow to the wood, is high in VOC's (only while still wet; the fumes disappear after curing), and requires mineral spirits or paint thinner to cleanup. Some people say that oil-based finishes look more realistic and water-based finishes tend to look more artificial, but water-based may be desired if you don't want to alter the color of the wood in any way or if you are sensitive to chemical fumes.