Finish Over Air-Dried Pine

      Soft resin and volatile natural solvents in softwoods will interfere with finishes unless the wood is kiln-dried at a high enough temperature to "set the pitch."August 17, 2009

Question
I am trying to make outdoor furniture with air dried red and white pine. I made a bench and finished it with Cabot Australian timber oil, but I had problems with the pitch running. I know kiln drying is the best solution, but I'm trying to avoid that if I can. Is there a product that prevents pitch from running that can then be covered with an outdoor finish? I even thought of using a propane torch on the knots before sanding hoping this may help the problem.

Forum Responses
(Finishing Forum)
From contributor Z:
Pitch /sap will eat through any finish in time. A torch will ignite the pitch/sap. I'm afraid your only option is to kiln dry. Slow heat with no flame. I believe the wood has to reach 160 degrees for a sustained time to dry and set pitch.



From the original questioner:
Contributor Z - thank you for the response. I guess I will look into having the lumber kiln dried.


From contributor B:
Have you tried using shellac painted over knots?


From contributor C:
A wash of 5-10% phosphoric acid followed by wood alcohol will d wonders in bringing the sap to the surface which the alcohol then will make inert.


From the original questioner:
I have not tried either the shellac or the phosphoric acid/wood alcohol, but I will give them a try.


From contributor C:
Actually, using them in conjunction should give you the best results. Be sure itís de-waxed shellac though either in flake form or Zinserís seal coat shellac.


From Gene Wengert, forum technical advisor:
The wood should be kiln dried (or heated) at 160 F or hotter for 24 hours or longer to evaporate the resin that is slightly liquid at room temperature. You can maybe rent a trailer, put the wood in it and then put in a heater (if you can do it safely to avoid a fire).


From contributor R:
If the lumber was kiln dried then the aforementioned suggestions would probably work. Shellac only works to help stop bleed through on painted pine knots etc. It will not stop pitch from running. As Gene said, heat it.


From contributor C:
You don't evaporate resin all youíre doing by heating is driving off the turpentine or other solvents. The "resin" is still left behind on/in the wood - therefore as I say if you use a five percent wash of phosphoric acid to coagulate the resin on the surface (which happens almost immediately) the wood alcohol will then reduce it to an insoluble state (a non reactive state).

The idea here is to destroy the activity of the sap on the same principle as alcoholic shellac is used before painting over it. This method has been used since the early 1900's. It's nothing new and is successful on pine or other oily woods such as teak or rosewoods. If you want to take the time to dry it you'll still have to deal with the resins especially in the knots. It does not stop the activity of the resin/rosin.



From Gene Wengert, forum technical advisor:
If you heat the wood, the pitch (maybe not the resin, but to me pitch and resin are the same) will be partially evaporated. This process has been used for many decades and in hundreds of commercial operations to avoid problems with pitch bleeding through.

From contributor C:
Yes pitch, sap, and resin are in this case spoken of as the same thing Gene. No offense intended - your correct that it will remove most of the volatiles from the resin/pitch itself but this does not stop its activity. Finishes can still in time flake off or from the presence of the sap/pitch/resin that's why an alcoholic barrier coat such as shellac is needed to stop this.

This was practiced since the late 1800's to overcome both this problem and the problems with all oily woods especially Brazilian rosewood. At that time mfg's were shying away from using it because of flaking or adhesion problems, it was then when it was found that the treatment I describe was first tried and introduced. The practice went on to be discovered by others to work just as well as on pine and other resinous woods of this type, especially turpentine pine. The drying will definitely help but is not the premier solution.



From Gene Wengert, forum technical advisor:
Thanks for the chemistry of the issue. However, I do know that heat alone has been very effective with finishes of the past, without alcohol. Regarding Brazilian rosewood, Spanish cedar, teak, and other hardwoods (leaf trees), hardwoods have gum and not pitch (or resin), so the heat treatment does not work at all for gum (that is, for hardwoods) and this has been known for a long time as well.


From Gene Wengert, forum technical advisor:
Incidentally, not all kiln drying goes to 160 F or hotter. Hence, not all kiln drying works to set the pitch.


From Gene Wengert, forum technical advisor:
Returning to your original concern about pitch running, heating will work to make any pitch that remains hard at room temperature. However, if the wood is dark and in the sun and the finish is dark and in the sun, the solar heating can soften the remaining pitch. For this reason, most operations prefer 180 F for the process that they call "setting the pitch."



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  • KnowledgeBase: Knowledge Base

  • KnowledgeBase: Finishing

  • KnowledgeBase: Finishing: General Wood Finishing

  • KnowledgeBase: Primary Processing

  • KnowledgeBase: Primary Processing: Air Drying Lumber

  • KnowledgeBase: Primary Processing: Kiln Operation


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