Thiko, I believe the product you are refering to is "Spar Varnish". This is different chemically from urethane. They used spar varnish for the finish on wooden boats for years and had to go back and redo them every few years. I never found primer, sealers or products made for the home wood work industry to be very suitable for actually water proofing anything. Any end grain really makes it hard to get a water tight seal by soaking in and sealing the wood pores. It seemed to me that a product that lays on the surface and covers the end of the wood cells (end grain) did a better job of keeping the water out. Keep in mind also that after the first coat of "whatever" on the wood, you are technically painting and adhearing to the first coat of whatever and not to the wood. As a test, take a few wooden blocks and measure the thickness and width accurately coat with your first coat of whatever, let dry completely. Throw them in a bucket of water over night and measure them the next day and see how much they changed size if any. If they changed, they soaked up some water and your coating didn't do its job. Length wont change much but thickness and width will. That is the easiest way to tell if your methods work or not.