There were posts that say to prime with sanding sealer? I hope not! Sanding sealer is a product used to apply over wood that has been stained that seals the wood and makes it easier to sand before applying the finish coats. It's only used as a first coat when you are appling clear finishes like varnish or polyurethane. As for painting six panel doors, you can prime with a high quality acrylic primer (tops and bottoms too) followed by two coats of finish paint. You may also want to look at masonite doors that come pre primed. 6 panel pine doors are not the best choice of doors to paint....in my opinion those are stain grade doors.

Locally, I'm not able to find samples of solid core "masonite" doors, only the hollow core junk. I've heard Masonite and Simpson are good. Any advice?