Sap from softwoods is like the famous escape artist Houdini. It can get through almost anything. Unfortunately, there’s no certain way to stop sap when it’s determined to ooze out of wood in large quantities. And the problem is always most pronounced with outdoor wood in sunny locations. The cycles of warming and cooling seem to drive the sap out more often and for longer periods of time than wood in constant shade. I’ve even seen sap continue to ooze from thick posts 10 years after construction. That’s what you see here to the right.
If your siding is due to be painted, use a stain blocking primer such as Zinsser B-I-N on the knots first, followed by paint. Even still, you can’t count on complete effectiveness in outdoor applications. If you don’t want to paint your siding, but would rather protect it so the grain shows through, then you can use stain block primer.
The best remedial solution I’ve found for situations like these involves scraping off hardened sap periodically, then wiping the area down with a rag soaked in turpentine. This is the best solvent to use for sap because turpentine is made from the same kind of pine trees that sap comes from. Perhaps it’s just my boyhood exposure to turpentine, but I really like the smell of it, too.