Knotting Solution: How to Stop Pine Knots Bleeding Through Paint
The UK guide to shellac knotting solution: why it matters, how to apply it, water-based paint compatibility, and what to buy. Tins from £7–£16, BIN from £23–£25.
Six weeks after your decorator finishes the extension, yellow-brown stains start appearing through the gloss on every pine skirting board in the room. They get bigger over winter as the central heating warms the timber. By spring, every knot is showing through three coats of white paint as a sticky orange blotch. The fix is to sand back to bare wood, apply knotting solution properly, prime, and repaint. That's a day's work per room, plus paint, on top of the original decorating bill. A small tin of knotting solution at £7 – £10 and twenty minutes per length of skirting would have prevented every bit of it.
What it is and what it's for
Knotting solution is shellac dissolved in methylated spirit (denatured alcohol). Shellac is a natural resin secreted by the lac beetle, refined into flakes, and dissolved in alcohol to make a brushable liquid that dries fast and leaves a hard, glossy, non-porous film. That film is the entire point. It physically seals knots in softwood timber so the resin trapped inside the knot cannot migrate out and discolour the paint above.
The job sounds niche but the consequences of skipping it are everywhere. Pine, redwood, and most other softwoods used for UK skirting, architrave, fascia boards, window boards, and door linings contain knots. Knots are where branches grew off the trunk, and they hold up to ten times more resin than the surrounding clear wood. Up to 40% of a knot's weight is extractives: sticky terpenes, resin acids (mainly abietic acid), and phenolic compounds. Warm a piece of softwood by a radiator and those compounds slowly liquefy and migrate to the surface. They oxidise on contact with air and turn yellow-brown. Over your white paint, they show as the orange stains every painter has seen on poorly prepared softwood.
Knotting solution stops this by creating a barrier the resin cannot get through. Standard primers do not. Water-based primers especially do not. This is the single most important thing to understand: you cannot substitute a normal primer for knotting on bare softwood and expect it to work.
What the British Standards say
Two standards govern knotting in the UK.
BS 1336:1971 Specification for Knotting is the product standard. It defines what a tin labelled "knotting" must do: minimum solid content, drying time, appearance of the dried film, bleeding resistance against pine resin, stain prevention, and resistance to white spirit. The standard has not been updated since 1971 because the chemistry hasn't needed updating. Any reputable UK knotting product (Wickes Trade Knotting Solution explicitly states "Conforms to BS1336") meets this specification.
BS 6150:2019 Code of Practice for Painting of Buildings is the painting code referenced by NHBC Standards Chapter 9.5. It mandates the sequence: knot treatment goes on bare timber, then primer, then undercoat, then topcoat. NHBC Standards Part 9.5 quotes this requirement directly: "All knots should be sealed using knotting applied by brush, or in the case of joinery part of the priming process." NHBC inspectors check for evidence of knotting at handover; missing it on visible softwood is a recordable defect.
NHBC then adds an interesting caveat in 9.1.10: resin can still exude from knots and discolour paintwork even where knotting was applied, listed as an acceptable natural variation at handover. Translation: even with knotting, very resinous live knots in poorly seasoned timber may eventually weep. This is why the trade fix for the worst knots is two coats of knotting plus a separate aluminium primer over the top.
Why water-based primers fail
Standard acrylic primer is mostly water with acrylic resin particles suspended in it. When you brush it onto bare wood, the water soaks into the timber, the resin particles fuse into a film, and the water evaporates. The film is good for sealing porous wood and creating an adhesion layer for the topcoat. It is bad at one specific job: stopping fat-soluble resin from migrating through it.
Two reasons. First, water-based primers have a porous polymer structure that fat-soluble extractives can diffuse through. Second, when you re-wet the surface (with the next coat of paint), water-based primers can soften slightly and "re-wet" any extractive that has reached the underside of the film, allowing it to migrate through to the topcoat.
Shellac works because it dries by solvent evaporation rather than coalescence. The result is a dense, non-porous film that does not re-wet under water-based topcoats. Resin acids and terpenes cannot diffuse through it. That is the only mechanism that reliably blocks knot bleed.
Shellac creates a physical barrier through solvent evaporation. Water-based primers form a porous film through coalescence. The two are not interchangeable on knotted softwood, no matter how many coats you apply.
Types of knotting product
Three categories of product all do roughly the same job, with different trade-offs.
| Product type | What it is | Best for | Watch out for |
|---|---|---|---|
| Traditional natural knotting | Orange/brown shellac dissolved in meths. The 'patent knotting' name on most older products. | Knots that will be finished with stain, varnish, or dark paint where the brown shellac won't show through. | Visible as a tan blob under white or pale topcoats if you apply it generously beyond the knot edge. |
| White (pale) knotting | Bleached shellac with white pigment. Same chemistry, lighter colour. | Knots under white or pale topcoats. The default choice for skirting and architrave painted in white satinwood or eggshell. | Slightly more expensive than natural knotting. Same drying times. |
| Zinsser B-I-N (pigmented shellac primer) | Shellac plus white pigment plus primer binders. Functions as knot blocker and primer in one. | Water-based paint systems, large areas of knotted timber, previously painted surfaces with bleeding knots. | More expensive per ml than dedicated knotting solution. Wrecks brushes (use cheap disposables). Strong meths smell during application. |
| Combined knot block primer-undercoat (Ronseal Knot Block, BIN Aqua) | Shellac-based primer-undercoat designed to replace the knotting plus primer plus undercoat sequence with a single tin. | DIY users wanting one product instead of three. Smaller jobs where a single tin justifies the price premium. | Less reliable than dedicated knotting on heavily resinous timber. Most professionals still spot-treat knots with traditional knotting first. |
For most extension work with painted softwood skirting and architrave, the practical choice is white knotting solution for the knots, then your normal primer, then undercoat and topcoat. If you're using a water-based paint system throughout, switch the knotting and primer steps for a single application of Zinsser B-I-N spot-applied to the knots and a separate acrylic primer everywhere else.
How to apply it properly
This is straightforward but every step matters. Skip any of them and the resin gets through.
Step 1: prepare the knot
Bare wood only. Knotting will not bond reliably to varnished, oiled, or previously painted surfaces. If you're treating timber that's been finished before, sand back to bare wood with 120-grit sandpaper first, then dust off thoroughly.
Before knotting, wipe each knot with a rag dampened with methylated spirit. This degreases the surface and removes any resin already sitting on top. If a knot is visibly weeping (sticky beads of fresh resin), warm it with a heat gun on low setting until the resin liquefies, then wipe it off with meths. Do not try to apply knotting over wet resin. Shellac will not stick to it and the resin will push straight through your barrier.
If a knot is loose (rattles when you push it), it's a "dead knot" and will eventually fall out. Knock it out with a screwdriver, fill the hole with two-part wood filler, sand flat, then knot the surrounding edge where the original knot was bonded to the trunk wood. The filler doesn't bleed; the wood ring around the knot still does.
Step 2: apply two coats
Use a cheap disposable brush. Shellac is very hard on bristles, and the brush will end up unusable after a few applications even with proper cleaning. Half-inch is enough for spot-treating knots on skirting.
Brush the knotting solution liberally over each knot, extending 10-15mm beyond the visible knot edge to catch the resin-rich wood immediately around it. Don't pool it. The shellac dries fast and excess will form a brittle skin on top of an unset layer beneath. Work each knot with three or four short brush strokes and move on.
Wait 30 minutes minimum between coats at room temperature. In cold or humid conditions, an hour is safer. Apply a second coat the same way. Most product datasheets specify two coats; manufacturers like Liberon claim one coat on their own product, but the BS 1336 compliance test is based on two-coat application and the trade default is two for a reason.
Step 3: let it cure before priming
Wait 30 minutes minimum after the second coat before applying primer. Shellac is touch-dry in 15 minutes but the film needs time to fully harden. Two hours is better if your schedule allows. Coo-Var's datasheet specifies overnight curing before topcoats; most other brands are happy with 30-60 minutes before primer.
Step 4: prime over knotting (don't skip the primer)
Knotting is a barrier, not a primer. It seals knots but does not seal the surrounding bare wood and does not provide the right adhesion surface for topcoats. You still need a normal primer over the entire piece of timber.
For oil-based paint systems (traditional gloss): use an oil-based wood primer. Knotting and oil primer are fully compatible.
For water-based paint systems (modern acrylic gloss, satinwood, eggshell): this is where things get awkward. Shellac knotting under water-based primer can show through as orange or pink patches if the primer doesn't bond properly to the slightly glossy shellac surface. The fix is one of:
- Use Zinsser B-I-N as the knot treatment instead of traditional knotting. B-I-N is shellac-based but pigmented white and formulated to take water-based topcoats.
- After knotting and a 24-hour cure, lightly abrade with 240-grit paper (just to dull the shellac sheen), dust off, then apply an oil-based primer. Water-based topcoats over oil-based primer is fine.
- Use a combined knot-blocking primer-undercoat (Ronseal Knot Block) that's designed for water-based topcoats from the start.
Spirit-based knotting under direct water-based primer is the second most common failure mode after skipping knotting entirely. It shows as orange or pink blotches that no number of topcoats will hide. If you're using modern water-based paint, choose Zinsser B-I-N or an oil-primer intermediate coat. Do not apply water-based acrylic primer directly onto traditional knotting.
Step 5: clean up
Methylated spirit cleans shellac off brushes, hands, and skirting boards. Water does not. Most decorators don't bother cleaning the brush at all because shellac brushes are so cheap to replace; the bristles harden after a few uses regardless of cleaning effort.
Decant a small amount of meths into a jar, work the brush in it for thirty seconds, wipe on rag, repeat. Do not pour meths back into the bottle once it's contaminated with shellac.
How much do you need
Coverage is the surprising part. A 125ml tin treats roughly 20-40 individual knots in two coats. A typical kitchen extension has 15-20 metres of skirting and 4-5 door sets with architrave. If the timber is decent quality with a knot every metre or so, you're looking at 15-25 knots in skirting plus 8-12 in architrave. One 125ml tin covers it comfortably.
For larger jobs, or for applying knotting on every length as a precaution rather than spot-treating visible knots, step up to a 250ml or 500ml tin. Coo-Var rates patent knotting at 12 m²/litre at full surface coverage; that's 3 m² per 250ml tin if you're brushing the entire face of the wood rather than just the knots.
If you're going the Zinsser B-I-N route instead, a 1L tin treats around 12.5 m² of full coverage or several hundred spot-treatments. For a typical extension, one 1L tin does the whole job (knots and primer on all softwood) with a margin.
Cost and where to buy
Knotting solution is one of the cheapest materials on the project, but the savings if you skip it are negligible compared to the cost of fixing the resulting bleed.
| Product | Size | Price (2026) | Where to buy |
|---|---|---|---|
| Liberon Patent Knotting | 125ml | £7.19 | Screwfix, Toolstation |
| Colron / Ronseal Knotting Solution | 125ml | £9.89-£10.29 | Screwfix, Toolstation, B&Q |
| Liberon Patent Knotting | 250ml | £12.29 | Screwfix |
| Bartoline Patent Knotting | 500ml | £10.29 | B&Q (cheapest per ml) |
| Wickes Trade Knotting Solution (BS 1336) | 250ml | £16 | Wickes |
| Rustins White Knotting | 300ml | £9.33-£15.75 | Decorating suppliers |
| Zinsser B-I-N (shellac primer-sealer) | 1L | £23.99 | Screwfix, Toolstation |
| Ronseal Knot Block (3-in-1 primer-undercoat) | 750ml | £20 | Wickes, Screwfix |
Typical 125ml tin: £7 – £10. Trade-size 250-500ml tin: £10 – £16. Zinsser B-I-N 1L (shellac primer alternative): £23 – £25. Ronseal Knot Block 750ml (combined primer-undercoat): £18 – £22.
For a typical extension's worth of skirting and architrave, the Liberon 125ml at
Knotting solution, 125ml tin (Liberon, Colron)
£7 – £10
Zinsser B-I-N shellac primer-sealer, 1L
£23 – £25
The Wickes Trade product at the top of that 250-500ml range is overpriced for what it is. The Bartoline 500ml is the best value per ml if you can find it in stock locally.
Why MDF skirting and architrave avoids the whole issue
MDF has no knots. It's pressed wood fibre with a uniform texture, no grain, and no resin pockets. Pre-primed MDF skirting and architrave (the kind sold by every DIY merchant and decorating supplier) skips the knotting step entirely and arrives ready for one coat of primer and topcoats.
For a typical extension, MDF skirting and architrave costs roughly the same as softwood (
MDF skirting board 119-145mm primed
£4 – £7
Pine skirting 119-145mm
£4 – £9
The trade-off: MDF can't be stained or oiled. If the brief is "natural pine skirting with clear varnish", you need real timber, and you need natural (orange) knotting because white knotting shows under clear finish.
Common mistakes
Skipping knotting because the joiner said primer was enough. A minority of joiners and even some painters dismiss knotting as old-fashioned. They're wrong, and the warranty data backs that up. Modern primers contain compounds intended to limit bleed (NHBC's 9.1.10 acknowledges this), but they do not block resin reliably. The dedicated shellac barrier is what works. If your joiner says it isn't needed, ask them to put that in writing and assume you'll be repainting the woodwork in six months.
Applying knotting to wet or weeping knots. Fresh resin on the surface prevents shellac from bonding. Wipe with meths first, or warm and wipe.
One coat instead of two. A single coat of shellac is often microscopically thin in places. The first coat soaks slightly into the wood; the second coat builds the actual barrier. Two coats is the BS 1336 standard for a reason.
Spirit-based knotting under water-based primer. Covered above. The orange-blotch failure mode. Use B-I-N or an oil-primer intermediate.
Using natural (orange) knotting under white paint. The shellac itself shows as a tan halo around each treated knot if the topcoat isn't fully opaque. White paint over white knotting hides everything; white paint over orange knotting needs three or four coats to mask the shellac colour.
Painting over already-bleeding knots without sanding back first. Knotting only works on bare wood. Once resin has bled into existing paint, you cannot apply knotting on top. The fix is sand back to bare timber, knot, prime, repaint. Or apply Zinsser B-I-N over lightly abraded existing paint as a stain-blocking primer (B-I-N's chemistry differs from pure knotting and can be applied over paint).
Storage and shelf life
Shellac dissolved in meths has a limited shelf life. Once a tin is opened, expect roughly 12 months of usable life before the shellac starts to thicken and lose its bleed-resistance. Store the tin tightly closed, upright, in a cool place. If the contents have noticeably thickened or formed a skin you cannot brush through, throw it out. A fresh tin at £7 – £10 is cheaper than the rework if old shellac fails to seal properly.
Shellac is flammable (denatured alcohol carrier). Don't store near heat sources. Disposal: take to a household waste recycling centre with hazardous-waste collection, not into general waste.
Where you'll need this
- Decoration - knotting solution applied to every visible knot in softwood skirting, architrave, window boards, and door linings before priming
Knotting solution is relevant during second-fix decoration on any extension, renovation, or refurbishment where bare softwood is being painted. The same product covers exterior work too: fascia boards, bargeboards, soffits, and timber cladding all benefit from knot treatment before exterior wood primer goes on. Anywhere bare softwood meets paint, knotting comes first.