Just curious about which water-based polyurethane you used. I've used the system 3 product on all 4 of the boats I've built and have been happy with the results...after I thinned it 1 part water to 1 part urethane. Also, humidity here in Ohio seems to be high enough for the stuff. I don't know how well it would work in a desert climate. Have you tried marking it up with permanent marker then sanding it with 220 on a ROS? The marker lets you know when it's been sanded down without going to far.