Donna lives in Georgian Bay, Ontario but has to share her deck with hornets that are attracted to the pine sap oozing out of her deck. What to do?
Pine sap will not stop oozing until it has all crystallized inside the wood. This process can be accelerated with heat, but even kiln dried and aged boards are not immune to problems from particularly sappy cuts of the tree. The good news is that sap can be cleaned up with turpentine, and water repellents will slow down its migration to the surface.
Shellac could block its movement, but shellac will not hold up to weather although it could be used under a good paint.
For clear finished surfaces I recommend the following maintenance until the problem just stops.
Scrub the sap off the surface completely with turpentine.
Heat the area for several minutes with a hot air gun to get some crystallization going -- though you will not have any effect deep down into the board. Then clean it again with turpentine to remove what may have come up with the heat gun.
Then spot-apply a good water repellent. Repeat this as often as the sticky stuff, or the hornets, show up.
What you are really doing is removing what is attracting the hornets while you are patiently waiting for the resins in the wood to completely crystallize. Because of the hornets, you may have to do this to all sides of the board -- wherever the sap comes out.