Unless they are primed, knot holes will reject painted finishes. They also tend to bleed through paint and are easily visible through light colours. If you wish to paint wood knot holes, be sure to employ the proper techniques. Condition the area with a stain-blocking primer base formulated to promote strong adhesion and lasting durability. Use a paintbrush engineered for rough surfaces, or you may have limited coverage. Coat the knot holes, using the correct technique, or you could end up with excessive runs and poor coverage.
Using the 180-grit sandpaper, sand splinters from the knot hole.
Clean sawdust from the knot hole using tack cloths.
Use the nylon-bristled brush to coat the knot hole with an acrylic stain-blocking primer. Dab the brush against the knot hole to ensure even coverage. Step back and wait thirty seconds for runs to appear. Smooth runs if they become evident. Allow the knot hole to dry for two hours.
Add a second coat to ensure the knot hole will not bleed through. Allow the knot hole to dry for another two hours.
Rinse the base primer from the nylon-bristled paintbrush with running water.
Coat the knot hole with latex paint with the nylon-bristled brush. Dab the brush against the knot hole to ensure even coverage. Step back and wait thirty seconds for runs to appear. Smooth runs if they become evident. Allow the knot hole to dry for two hours.
Use acrylic latex paint on knot holes located on exterior wood surfaces.
Never paint unprimed knot holes because peeling will result. Don't use a polyester brush on knot holes. That can cause coverage problems. Knot holes require a stain-blocking primer. Don't use latex primer, or you will be able to see the knot holes through the painted finish.