A.Paul R. Fisette, director of the wood technology program at the University of Massachusetts in Amherst, responds: The sap in the knotholes softens and begins to runs as it is heated by the sun. Sap is composed of liquid and solid materials. If it is heated to 160°F while it is being kiln-dried, the volatile liquid substances will flash off, leaving the solids behind in a hardened state. Once the sap has set, it will never run again. This doesn’t help you now, however, since the wood in your deck was obviously not kiln-dried at a high enough temperature during production.
There are a couple of options. You can wait it out; sooner or later the sap will stop running out of the wood. In the meantime, you can clean it up with turpentine. Or you can try to set the sap in the knots using a heat gun. Just be careful not to start a fire or singe the wood.
to read the full article.