We use the pigmented shellac in the aerosol can on knots and sap, then prime the whole thing with a stain blocking primer. We like to prime the back too, because today's knotty pine is prone to warp, esp. on porch ceilings where the topside can get really hot/dry under the roof.
Casey

Thank you sombreuil. Any particular stain blocking primer you like?

We used to exclusively use oil Fresh Start stain blocking primer by BenMoore, which I thing is an purple label, but we switched over mainly to "Suprime" by Pratt & Lambert so we don't have to wait a day (or more) for it to dry. The P&L isn't a stain blocker, but if you did a good job with the Kilz spray, and you want to install right away, coat the backs with the P&L, install, then if you're still worried about any bleed thru, you can use the Moore Alkyd Fresh Start purple label on the face side, because there wouldn't be any drying time constraint since it's installed and out of the way. You could also use any of the Kilz stain blockers, since they are less costly and available at more places.
Casey