Priming wood and wood products is of particular importance for several reasons:
1. Change in temperature causes wood to shrink, expand, and warp, which can stretch the paint film beyond its tolerance.
2. Water absorption into the wood either from exterior sources or from the paint itself will cause the wood to expand while wet and contract as the paint dries. This could cause the paint to dry unevenly, which can trigger cracking or cratering
3. Any sap still present in the wood can inhibit proper curing in the paint, and can stain light colors.
4. Large knots in wood should always be sealed with proper wood sealer before being primed with an acrylic product.