I have a 3″-thick slab of pine (I think) that came from Canada, I’m told. It was milled at least 15 years ago and has been in my workshop for the past 10 years. I’m sanding it to prepare to use it for a coffee tabletop. It is still oozing what appears to be a little sap. I’ve scraped off all that I can, and when I sand the sappy area you can see it turn dark on the slab and it builds up on the sandpaper. What would you recommend as a sealer before finish is applied? I plan to use spar urethane as a finish.
– Granville Jones
Trying to seal liquid sap is an exercise in futility. Spar urethane (a misnomer, by the way) is probably exterior urethane, and while it would undoubtedly cure over the sap pockets, do you really want active, oozing sap under your cured finish? That can’t end well.
The traditional material for sealing sappy knots is called “knotting” and is made of thick shellac. It works moderately well for a little while but ultimately fails. I’ve seen sap make its way through just about every clear wood finish, thick paint and even through vinyl “contact paper” drawer liner material. There is a way to “set” the sap by heating the wood past the point where sap crystallizes, but it’s probably not practical on your thick pine slab. Personally, I would consider using something else for a coffee tabletop. After all, even the best finish has its limitations.