Stearic Acid is found in in Vegetable oils and in checking Linseed oil does have between 4 & 5% though much lower than in Animal Fats. Polyurethane is the kiss of death to coin a phrase for gun stocks because it dries so hard and brittle that it will not allow the natural seasonal movement of wood without cracking also not allowing the passage of water vapour in or more importantly out, in fact the wood is held in an impervious box. Though I do not have any idea of the formulation of sanding sealers though I have been using them for some twenty five years now with no problems covering them, though I will not use any finish containing Polyurethane for Wood.
