The shellac will form a hard shell (hence the "shell" in the name) that the pitch can't come through.
This is the idea I was trying to explain. I my experience, when the pitch gets warm and turns from a crystaline form to a liquid, it also expands causing pressure within the wood, resulting in the "oozing" effect of the pitch. I have dismantled 100+ yr old sawn Pine timber-frame barns that were originally framed green. Once the frames were covered with a roof and siding, the interior beams were sheilded from the sunlight, and many of the knots stopped releasing pitch. So, when I re-expose beams that haven't seen "the light of day" for over 100 yrs to direct sunlight, they start releasing pitch again...almost as if someone had pressed the "pause button".
Rooster
