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.
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.
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.
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.
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.
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.