Why are knots showing through my paint?

A knot is where a branch was, and it is dense end grain full of resin. Warm it — with a radiator underneath, sun on a sill, or simply a heated room in winter — and the resin becomes mobile and moves outwards. Ordinary primer and topcoat are permeable to it, so it travels through and appears at the surface as an amber or brown ring, sometimes with a sticky bead in the middle. It is seasonal, it recurs, and it is entirely preventable at the right moment.

What you'll need

• Shellac knotting solution
• Abrasive paper and a scraper
• Wood primer and your topcoat
• Methylated spirit, for cleaning brushes used with shellac
• A heat gun or hot air stripper, for a knot still weeping resin

Step by step

1. Confirm it is resin and not something else.Resin bleed shows as a discrete brown or amber circle or oval exactly over a knot, sometimes glossy or slightly sticky, and it gets worse when the surface is warm. A brown patch not centred on a knot is a different problem — a water stain, tannin bleed or rust from a fixing.
2. Sand back to bare wood over each affected knot.Scrape and sand the paint off the knot and about 20 to 30mm beyond its visible edge, because the resin-rich wood extends past what you can see. Wipe the surface clean of dust and any surface resin with methylated spirit.
3. Deal with a knot that is actively weeping.If liquid resin is still coming out, sealing it now traps a wet knot under the barrier. Warm the knot gently with a heat gun to draw the resin out, scrape it off as it appears, repeat until nothing more comes, then let it cool and clean it. Alternatively cut the knot back a millimetre or two and fill.
4. Apply two thin coats of knotting solution to the bare wood.Shellac knotting is the traditional and still the best barrier. Brush it thin over the knot and the surrounding area, let it dry — which is minutes, not hours — then apply a second coat. One coat is often not a continuous film, and a barrier with a gap in it is not a barrier.
5. Get the barrier in the right place.Knotting goes on the bare timber, underneath the primer. Applied over primer it fails, because the primer is the layer the resin travels through and the resin simply comes up behind the barrier and around it.
6. Prime, then rebuild the coats.Wood primer over the knotting and the surrounding bare timber, then undercoat if your system uses one, then the topcoat. The patch will show slightly against aged paintwork, so on a visible surface take the topcoat to a natural break — the full face of a door, the whole length of an architrave.
7. Do it properly on the next new timber you paint.Every knot on every piece of bare softwood gets knotted before priming, as a matter of routine. It costs a few minutes on a new skirting run and saves stripping it back in eighteen months.

Tips

• Shellac knotting dissolves in methylated spirit, not water or white spirit. Clean brushes in meths, or use a cheap brush and throw it away.
• A pigmented shellac-based stain-blocking primer will do the same job and gives you a primer coat at the same time. It is a good choice on a run of new skirting with many knots.
• Knots on a window sill or a door in direct sun bleed hardest, because those get warmest. Give them a third coat of knotting.

Common mistakes

• Painting more topcoat over the stain — Resin migrates through paint. You are adding another permeable layer, and the same ring reappears within a season.
• Applying knotting over the primer — The barrier has to be between the resin and everything else. Over the primer it is on the wrong side of the layer the resin travels through.
• Sealing a knot that is still weeping — Wet resin under the barrier keeps moving and eventually pushes through. Draw it out with heat first.

If it doesn't work

Questions people ask