What is the best undercoat for wood?

Best undercoat for wood depends on whether wood is interior or exterior and whether it has knots or tannins, but generally oil based or shellac based primer is best for bare wood, especially with knots, to seal and block tannin bleed. For bare interior wood like pine, cedar, or wood with knots, best undercoat is shellac based primer like Zinsser B-I-N which blocks tannins, knots, and sap bleed that would show through water based primer, or oil based wood primer like Zinsser Cover Stain or Benjamin Moore Fresh Start. For bare exterior wood, best is exterior oil based wood primer or acrylic wood primer designed for exterior that seals wood and resists moisture, such as Benjamin Moore Fresh Start exterior wood primer or Sherwin Williams Exterior Wood Primer. For previously painted wood in good condition, you can use general purpose primer undercoat or combined primer undercoat. For wood that will be painted with water based topcoat, you can use water based acrylic wood primer for low odor, but for knotty wood, oil or shellac is superior. For MDF, use MDF primer. So best undercoat for wood is stain blocking primer that seals grain, blocks tannins, and provides adhesion, with shellac or oil based being most effective for bare and knotty wood, and acrylic wood primer being good for general purpose low odor interior.