Thompson's water seal is junk do not use it. I Used spar varathane, something similar will work, I have had good results with spar varnish. I put Thompson's on my deck and you could not even tell after one winter it was useless and I re did it with behr much better and cheaper than varathane urethane option for the trailer. Varathane urethane for side rails.
The road to Hell is paved with good intentions. KARL MARX