When you have to paint bare wood, it is always advisable to start off by sealing the knots with a shellac based sealer. This prevents resin from a weepy knot staining through the top coats of finished paintwork. Knots may leak wherever there is heat to activate the resin - think super centrally-heated houses and south-facing exteriors.
It is a time-old issue but solved 95% of the time with 2 quick licks of shellac.
One other factoid is that you shouldn't use shellac-based sealers like Zinsser BIN as a general wood primer on exteriors . "It's brittle" is the stock answer. True, but if you carry on asking questions, the science is kind of contradictory.
I dive a bit deeper into shellac, one the oldest paint products in the book, in this article! Yawn!
