GF Milk Paints are self-sealing so it is not necessary to use primer. You may use a stain-blocking primer if you need to seal knots, cover grain, paint light or white color on dark wood, or if you're painting a wood that is known to bleed, such as pine or oak. In those cases, we recommend using General Finishes Stain Blocker. Kilz or Zinsser primers are options as well.