Sealing a wood item does not prevent the wood itself from absorbing water, but if you are coating with our protective coatings, it should assist in limiting the ground contact absorption. Remember… wood that absorbs water and cannot dry tends to accelerate wood rot. Always use proper wood for the intended use and ensure the wood is completely dry to provide proper adhesion.
Generally, it is best to simply coat the top and sides of wood (leaving the bottom un-coated). This allows the wood to dry from any air or other water absorption.