A Primer That's Easy to Sand
If your goal is an ultra-smooth topcoat on a bare wood with visible pores, like pine, your best bet is to prime the wood first. A good primer will fill in the pores, and a good sanding will smooth out all the brushstrokes. Trouble is, many primers gum up sandpaper like nobody's business. Here's one that doesn't.
Benjamin Moore's Fresh Start Enamel Underbody Primer is specifically made to sand easily. It's an alkyd (oil-based) paint, so it flows and levels extremely well. It's ideal for doors, cabinets and trim that will be painted with a gloss or semigloss enamel. You'll have to wait overnight before sanding and recoating, however.