Dave, I took the boards out of my pickup and took them to the guy who built all my cabinets when I was home builder.
He ran them through his planner and took off enough to barely get the old finish off. I measured it and doggone it, you couldn't tell he took any off!! I then redid mine using the Helsman Spar Urethane finish and it still looks nice but I must admit it stays in the shop most of the time.
Nevertheless, the planning sure did the trick. I had a couple of small low places about the size of a silver dollar and a paper dollar but sanded those by hand so as to not cut anymore off. Oh, and it was Oak.
