Have used Balsarite for a couple decades, under monocote and under 21st Century fabric and film. No problems with bubling, but I only use it around engine openings or other areas where fuel contamination tends to lift the covering. BUT, I only use it very sparingly- very light coats.

Also- Balsarite has been a real help in re-covering old planes that have years of soaked in fuel in the balsa.