Decorator's Caulk: What to Buy, How to Apply It, and Why Paint Cracks Over It
The complete UK guide to decorator's caulk: where to use it, which brands to avoid, proper application technique, and how to stop paint cracking. From £1-5 per tube.
Your decorator has just painted the new extension and it looked perfect. Two weeks later, dark hairline gaps have opened along every skirting board, around every architrave, and at the ceiling line. The walls haven't moved. What happened is that nobody caulked before painting. Those gaps were always there, hidden by shadow and wet paint, and now they're the first thing you notice in every room. Filling them after the fact means careful masking, repainting, and hoping the colour match is close enough. A tube of caulk before the final coat would have prevented all of it.
What it is and what it's for
Decorator's caulk is a flexible acrylic filler that comes in a cartridge and is applied with a caulking gun. It fills the small gaps (typically 1-6mm) that exist between timber trim and plaster surfaces: the top edge of skirting boards against walls, both edges of architrave around door frames, the junction between coving and ceiling, and internal window reveals where plaster meets the frame.
The critical difference between caulk and silicone sealant is paintability. Caulk dries to a matt finish and takes emulsion or gloss paint directly. Silicone cannot be painted. That single distinction determines which product goes where: caulk for any junction that will be painted, silicone for any junction that gets wet (baths, showers, kitchen splashback edges).
Caulk is not a structural filler. It can't be sanded, it's not waterproof, and it won't bridge gaps wider than about 6mm without sagging or cracking. For holes in walls, use a proper powder filler. For bathroom junctions, use silicone. Caulk does one job, but it does it well: it creates clean, sharp painted lines at every junction in a room.
There's no building regulation or British Standard governing decorator's caulk. It's a trade consumable used at the decoration stage. But it's the difference between a professional-looking finish and one that screams "DIY weekend."
Types and variants
Most homeowners will only need standard interior decorator's caulk. But there are variants worth knowing about, because the wrong type in the wrong place will fail.
Standard interior caulk is what you need for 90% of decoration work. It's water-based acrylic, dries in 1-2 hours, and costs under £3 a tube.
Exterior caulk costs roughly three times more per tube. It's formulated to resist rain once cured and will adhere to slightly damp substrates. You won't need this inside your extension.
Hybrid sealants sit between caulk and silicone. Products like CT1 and Soudal Fix-All Flexi are both paintable and flexible enough for joints that move, particularly window reveals in newly built walls where settlement causes standard caulk to crack within months. They cost more but they're the right product for that specific joint.
Standard decorator's caulk is the wrong choice for window-to-plaster junctions in new builds. Settlement in the first 12-18 months pulls the joint open. Use a paintable hybrid sealant for window reveals, or expect to re-caulk annually.
How to work with it
What you need
A caulking gun (skeleton type, about £5 – £8). A sharp utility knife or Stanley knife to cut the nozzle. A small bucket of warm water. A clean sponge or lint-free cloth. Paper towels. Masking tape if you're a beginner.
The right sequence
This is where most guides fail you. The order matters and getting it wrong causes the most common caulk problem (paint cracking).
Plaster must be mist-coated before any caulk goes near it. Bare plaster is porous and will suck the moisture out of caulk before it cures properly. The caulk peels away weeks later. Apply your mist coat (emulsion thinned 50/50 with water) to all new plaster first. Let it dry completely. Then caulk.
The full sequence for a room in a new extension:
- Plaster dries fully (4-6 weeks for new skim)
- Mist coat all walls and ceiling
- Fix skirting, architrave, and any coving
- Undercoat/prime all timber with an oil-based primer
- Caulk every junction
- Allow caulk to cure (24 hours is safest)
- Apply first wall paint coat
- Apply gloss/satin coat to timber
- Apply second wall paint coat
- Final timber coat
Some professional decorators apply caulk after the final wall paint coat, using a hairline bead that sits on top of the paint surface rather than being painted over. This prevents the cracking problem entirely because the paint film and caulk move independently. It requires a very steady hand and a very thin bead. If you're confident, it gives the cleanest result.
Application technique
Cut the cartridge nozzle at a 45-degree angle, about 5mm from the tip. A common beginner mistake is cutting too far up the nozzle, which creates a bead that's far too thick. You can always cut more off. You can't put it back.
Load the cartridge into the gun with the nozzle facing forward and the plunger engaged. Hold the gun at roughly 45 degrees to the joint. Pull the trigger smoothly while drawing the gun steadily along the gap in one continuous motion. Work in manageable lengths of about 1-1.5 metres at a time.
Now the important part: tooling off. Dip your finger in water (keep a small container next to you) and run it along the bead in one smooth pass. Press gently. The goal is to push the caulk into the gap and wipe away the excess, leaving a thin, concave line. One pass. Don't go back and forth or you'll drag caulk out of the joint and create a mess.
Wipe your finger on a damp cloth after each pass. Move to the next section. Speed matters here because caulk starts to skin over within 10-20 minutes. Once it skins, you can't smooth it without tearing the surface.
If masking tape gives you confidence, apply it along both sides of the joint before caulking, leaving 2-3mm exposed either side of the gap. Apply the caulk, smooth it, then peel the tape immediately while the caulk is still wet. Leaving tape until the caulk dries will tear the bead when you remove it.
Deep gaps
For gaps wider than about 6mm, don't try to fill them with caulk alone. It will sag, shrink dramatically, and crack. Pack the gap first with a backer rod (a foam strip from any builders' merchant) or coving adhesive, let that set, then apply a thin caulk bead over the top.
Caulk shrinks by roughly 4-15% as it dries. On a 3mm deep fill, that's barely visible. On a 10mm fill, you'll have a noticeable groove. For fills deeper than about 3mm, expect to apply a second thin bead once the first has dried.
Why paint cracks over caulk (and how to fix it)
This is the single most discussed problem on every UK DIY forum. Paint cracking or crazing over caulk is so common that some homeowners assume it's inevitable. It isn't. There are four causes.
You painted too soon. Product labels say "paintable in 1 hour." That's technically true for a thin bead under 1mm in a warm, dry room. In practice, a 3mm bead in a new-build extension in February is not cured in one hour. Wait 24 hours. Seriously. The one-hour claim is for ideal conditions that rarely exist on a building site.
The bead was too thick. A fat bead of caulk shrinks as it dries. That shrinkage pulls the paint film apart. The solution is always a thin bead. Fill the gap, don't build a mountain.
You used cheap paint. Vinyl matt and contract matt emulsions are the worst offenders for cracking over caulk. They're less flexible than trade-quality paints. If you're painting over caulked joints with matt emulsion, use a trade brand (Dulux Trade, Johnstone's Trade) rather than a £12 value tin.
You caulked bare plaster. The mist coat is not optional. Caulk on bare plaster loses moisture too fast, cures poorly, and the paint system fails at the bond between caulk and plaster.
If cracking keeps happening despite doing everything right, apply a thin coat of shellac-based primer (Zinsser BIN is the standard trade recommendation) over the caulk before emulsion. This seals the caulk surface and gives the emulsion a stable base. Professional decorators use this routinely on new builds.
What to buy
The graph lists decorator's caulk at £1 – £5 but brand choice matters more than price on this product. Some budget caulks work well. Others are actively bad.
The brands that work
No Nonsense (Screwfix own brand) is the budget pick. The 310ml tube at about TBC or the 380ml at TBC are both perfectly adequate for standard gaps. It's not the smoothest to tool off but it cures reliably and doesn't bleed into paint.
HB42 Ultimate is what professional decorators reach for. It's a modified polymer (not pure acrylic) with very low shrinkage. Around £5 – £6 per tube. If you're caulking an entire extension and want minimal problems, this is the one.
Everbuild Everflex 125 is a solid mid-range choice. About £2 – £3 per tube. Fast-drying, reliable, and widely stocked at Toolstation. Some reports of yellowing after a few months when used under certain paints, so stick with white surfaces.
Geocel Painters Mate sits between budget and premium at around £0 – £0. Widely available at Toolstation, Wickes, and Screwfix. Smooth to apply and low odour.
The brands to avoid
Soudal standard shrinks badly and bleeds (releases a yellowish discharge that stains through paint). Forum consensus across multiple independent threads is consistently negative. The Soudal Turbo variant may be better, but the standard product has a poor reputation among trade decorators.
Wickes own-brand leaches into paint and causes crazing. At TBC per tube it's cheap, but you'll spend more on repainting than you saved.
How many tubes do you need?
A standard 310ml cartridge covers approximately 12 linear metres of a 3mm bead. For a typical extension room (say 4m x 5m):
- Skirting: approximately 18 linear metres (perimeter, less doorway)
- Architrave: approximately 10 linear metres (two edges per doorway, two doorways)
- Ceiling line (if no coving): approximately 18 linear metres
- Window reveals: approximately 6 linear metres per window
That's roughly 50 linear metres, so 4-5 tubes for one room. Buy six. You always need more than you think, and a partially-used tube doesn't store well.
To preserve a partially-used cartridge, push a screw or nail into the nozzle and wrap the end tightly in cling film. Stored upright in a cool place, it'll last 4-6 weeks. Without sealing, expect the nozzle to block within days.
Cost and where to buy
Decorator's caulk is stocked everywhere. Screwfix, Toolstation, Wickes, B&Q, Travis Perkins, and Jewson all carry multiple brands. Prices start from under £5-£9 per tube for own-brand and budget options, rising to £5 – £9 for premium or specialist products.
For a whole extension (3-4 rooms plus hallway), budget for 15-20 tubes. At the budget end (No Nonsense), that's about £20 – £30 total. At the premium end (HB42), it's £20 – £30. In the context of a build costing tens of thousands, this is not a product to economise on.
Buying in bulk saves money. Soudal 12-packs at Toolstation come in around £18 (though given the brand's reputation, that's a false economy). Polycell Trade 380ml 12-packs at about £18 are a better bulk buy. No Nonsense multi-packs at Screwfix drop the per-tube cost below £18.
Alternatives
Silicone sealant is waterproof and flexible but cannot be painted. Use it around baths, showers, and external window frames. Not an alternative for interior decoration work.
Hybrid sealants (CT1, Soudal Fix-All Flexi) bridge the gap between caulk and silicone. They're paintable, waterproof, and more flexible. Use them for window reveals in new builds where settlement is expected. They cost 3-5 times more than standard caulk but they won't crack at high-movement joints.
Flexible acrylic filler (brands like Dunlop Flexible Filler) is similar to caulk but sold in tubs for application with a filling knife. Useful for slightly larger surface repairs that need flexibility, but not as neat as a caulked joint for skirting-to-wall work.
Powder filler (Polyfilla, Toupret) is for holes and dents in walls. It dries rigid, can be sanded smooth, and paints well. But it has zero flexibility and will crack at any junction where two different materials meet and move independently. Never use powder filler instead of caulk at a skirting joint.
Where you'll need this
- Windows and Doors - filling the gap between window/door frame and plastered reveal before internal painting
- Decoration - caulking all skirting, architrave, and ceiling line junctions before final paint coats
Common mistakes
Caulking before the mist coat. This is the most common failure. Bare plaster absorbs moisture from the caulk, weakening the bond. The caulk peels off weeks or months later, pulling paint with it. Mist coat first, always.
Cutting the nozzle too wide. A fat nozzle deposits a fat bead. A fat bead shrinks visibly and is hard to smooth cleanly. Cut small. You want a 2-3mm opening, not a 6mm one.
Painting the same day. The "1-hour" claim on the tube is marketing for ideal conditions. In a new-build extension with fresh plaster and limited heating, cure time is much longer. Wait 24 hours. Your decorator might push back on this because it adds a day to the job. Hold firm.
Do not use decorator's caulk in wet areas. It is not waterproof. Bathrooms, shower rooms, and behind kitchen splashbacks all need silicone sealant. Caulk in a bathroom will absorb moisture, swell, discolour, and eventually harbour mould.
Using caulk as a general filler. Caulk fills gaps at junctions. It cannot fill holes, dents, or screw holes in walls. It can't be sanded. If you try to use it as a surface filler, you'll see every repair through the paint. Use powder filler for surface repairs, caulk for junction gaps.
Forgetting window reveals. Every guide mentions skirting and architrave. Fewer mention the gap between plastered window reveals and the window frame itself. This joint needs attention too, though in a new build you should use a hybrid sealant rather than standard caulk here.
Working in cold conditions. Most caulk products specify a minimum application temperature of 5 degrees C. Below that, the caulk won't cure properly. In an unheated extension during winter, check the temperature before caulking. If it's too cold, wait for a warmer day or get temporary heating running.