How to Waterproof Treated Lumber Buried Underground

Written by scott knickelbine
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How to Waterproof Treated Lumber Buried Underground
Poly resin wood hardeners can keep pressure-treated lumber from rotting underground. (lumber 1 image by jimcox40 from

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.

Skill level:

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Things you 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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  1. 1

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

  2. 2

    Pour the wood hardener into a can or paint tray.

  3. 3

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

  4. 4

    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.

  5. 5

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

Tips and warnings

  • 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.

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