It will be interesting to see how long it will keep a seal.
In a humid environment the wood will expand and crack the urethane. also I would have keep the delution ratio 10 to 1 (min.) applied to the wood as dry as possible to let the wood suck the sealer into the fibers as deep as possible (the end grain whill drink and drink, filling the pores). Then thicken it a little to finish it off. The IDEA condition is to put in a pressure pot to "pressure treat",i.e. drive the solution as deep into the wood as possible.(I use pressure tanks in my work)
I would possible be inclined to use an epoxy called "Get Rot" found at marine stores for boat repare. It is inert when cured and it has been used for yrs. by a Bonsai friend of mind to seal dead wood on his trees (some of this wood a 100 yrs ) The trees are going thru heat, cold, rain, ice etc.
nuff said