Firstly, when do you need to prime? If you're using a white or pale shade of paint and your surface has the potential to bleed, then you need to use a colour blocking primer. A colour blocking primer will create a chemical barrier, stopping any stains or tannins from seeping through your beautiful paintwork.
Without this if you paint mahogany white it'll turn pink, those knots in pine will show right through, also oak will get nasty yellow streaks! We don't want that!
Now historically you'd need a shellac based primer to do this job, but the clever chemists at General Finishes have created a waterbased formula that will do this job...omg that's so much more convenient!
Stain Blocker is General Finishes' revolutionary water-based stain-blocking primer for interior projects. Stain Blocker can be used to protect your finish from stain, dye and wood tannin bleed-through. Brush, roll or spray over an existing finish, raw wood or MDF as a base coat for light-colored paints such as General Finishes Milk Paint and Brushable White Enamel. We recommend that you apply 2 coats of Stain Blocking Primer.
This is not an adhesion primer, as with all paints and projects your surface will require cleaning and sanding before application of stain blocker.