How to Waterproof Treated Lumber Buried Underground

Updated April 17, 2017

Keeping wood buried underground from rotting presents a challenge. Wood under these conditions is subjected to contstant moisture, bacteria and insects. Even pressure-treated wood will rot eventually if buried. Fortunately, wood hardening products exist on the market that will greatly prolong the life of buried wood. These resins soak into the wood and bond with the wood fibres, producing a rock-hard and completely waterproof surface.

Clean any loose wood, dirt or oil from the pressure treated lumber.

Pour the wood hardener into a can or paint tray.

Apply a coat of hardener to the entire surface of the wood with a bristle brush.

Apply two or more additional coats to the wood, waiting only a minute or two between coats, to build up a glossy surface on the wood.

Allow the hardener to cure at least two hours before installing the wood below ground.


Use a disposable brush and can when working with wood hardener; because the compound dries so quickly, it is difficult to clean these application tools for reuse. You can also use two-part epoxy resins to harden wood; these are somewhat less expensive, but require careful mixing and rapid application before they set.


Wood hardening formulations contain volatile solvents that are harmful if inhaled in high concentrations. Always work with these products in areas with good ventilation. Be careful to do any woodcutting or installation of fasteners or hardware in the wood before applying the hardener.

Things You'll Need

  • Pressure-treated lumber
  • Wood hardening compound (e.g. Minwax High Performance Wood Hardener)
  • Disposable bristle brush
  • Metal can or paint tray
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About the Author

Scott Knickelbine began writing professionally in 1977. He is the author of 34 books and his work has appeared in hundreds of publications, including "The New York Times," "The Milwaukee Sentinel," "Architecture" and "Video Times." He has written in the fields of education, health, electronics, architecture and construction. Knickelbine received a Bachelor of Arts cum laude in journalism from the University of Minnesota.