Originally Posted by Will22
It's tannin acid.
Pine does not contain tannins, like redwood and cedar do. It bleeds sap from knots in the wood. As stated previously, a shellac base coat will block this out before painting.
Sorry Will, that's just not true. How would sap from knots discolor the whole side frame of OP's window?
All pine contains tannin. White pine contains less tannin than yellow pine. I've always known pine to be a bleed problem from tannin, and a quick google search of pine and tannin
only proved it. They actually make products specifically from the tannin of pine.