I am making sections of cedar tree trunks into sitting stools for the garden. They are in all stages of dryness and I strip the bark off and sand to make smooth. I have no trouble with the sides, but sometimes the top gets a few "pimple like bubbles" after drying. I am using exterior shellac and applying it in the garage, which is very hot right now. What is causing the bubbles? Should I switch to another product? Someone suggested to just use linseed oil.