I refinished my oak hardwood floors about 5 years ago. I used Minwax Clear Polyurethane and I know I wont be using it again. After just 5 years, no pets, no kids, the finish is dull and already worn down in several areas. I had 3 coats on there and they looked great for about the 1st 3 years. Now they need to be re-done again already ! My question is.... do I have to re-sand down to bare wood or can I just go on top of the current finish with 2 coats of a better polyurethane ?