Don, been the bubble route and really frustrating. First set of wood I did was primed and painted two coats of the Rustoleum AFTER applying several coats of Thompsons as per a local wood worker who "had experience" doing bedwood. Sat in the shop for about 10 days drying or so I thought then one day I think, Hmmmm! lets put it in the sun to bake. 4-5 hours later I had blisters as big as your had on every board. Learned later, on this board Thompsons is a no-no. Second set of wood prime and two coats of Rustoleum and NO Thompsons. Same procedure no blisters after 3 years so far. Don't know anything about Penetrol mentioned but personally I would be leary of additives until I I ran a test board.
