Thompsons is a 'stand alone' sealer, so the paint had nothing to hold on to, hence blistering in the force-dry you cite -
Penetrol is more a \'wetting agent\' that helps the paint hang on and penetrate deeper into the wood - exactly opposite to Thompsons, which might do an acceptable job alone for a 'natural wood finish', but I think it's over-rated
you can decide to use Penetrol or not, but I've found it excellent for painting anything w/ harsh exposure, and have used it in a number of situations
Bill
