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How to Treat Pressure-Treated Board Knots

Updated July 20, 2017

Knots are a natural feature of pressure-treated wood, occurring where a branch was connected to the trunk of the tree and showing as rounded features within the grain. Knots present problems when the lumber is used in building projects. Their irregular surface can lead to an uneven finish, and they may continue to leak sap after pressure treatment which can bleed into paint causing unsightly stains. Treating knots in pressure-treated wood before use can prevent these problems.

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  1. Sand the surface of the wood where the knot is located using a coarse grit sandpaper. Rub the sandpaper along the wood applying a firm pressure in the direction of the wood grain. Continue sanding until any irregularities in the surface of the wood are smoothed out. Repeat using a medium-grit sandpaper for a more refined finish. Wipe away any dust created using a clean wet rag. Allow to dry.

  2. Paint a generous application of liquid shellac or knotting lacquer over the knot and surrounding area using a soft-bristled paintbrush. Allow to dry thoroughly before applying a second coat. Leave until completely dry. This will form a seal preventing any sap from bleeding into surface paint.

  3. Cover the knot and surrounding area with stain-blocking primer. This should be painted on using a soft-bristled brush and allowed to dry. While one coat is usually enough to prevent the outline of the knot from showing through future paint jobs, you may want to apply a second coat for extra measure once the primer is dry.

  4. Check for any cracks around shrivelled knots or areas where knots are recessed into the wood. If these are present, use an all-purpose wood filler and allow to dry before re-sanding and painting.

  5. Tip

    To save time, an electric sander can be used in place of manual sanding.


    Loose knots will only get looser as the wood dries. If a knot seems unstable, the wood may need replacing in order to avoid the appearance of a hole.

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Things You'll Need

  • Coarse-grit sandpaper
  • Medium-grit sandpaper
  • Clean rag
  • Soft-bristle paintbrush
  • Liquid shellac/knotting lacquer
  • Stain-blocking primer
  • All-purpose wood filler

About the Author

Jenni Potter is a qualified senior journalist and newsroom veteran with 10 years experience on weekly and daily titles across the UK. She now works as a freelance copywriter specializing in SEO web content. Potter trained at the Trinity Mirror School of Journalism and holds a Foundation and Diploma in Journalism.

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