Acrylic sealant for a window frame
Updated: 23 October 2026
The cartridge will either seal a still gap you can paint or glue a sash that should open. Acrylic sealant belongs on the static joint between frame and wall, usually the internal reveal, and only when that face stays dry enough to paint. It does not replace a brush seal, and an internal-only cartridge does not seal the weather face. The listings are on the [acrylic sealant comparison](/hub/fillers-adhesives-and-sprays/acrylic-sealant). A purchase through our links can earn a commission at no extra cost to you.
The cartridge will either seal a still gap you can paint or glue a sash that should open.
The cartridge will either seal a still gap you can paint or glue a sash that should open. Acrylic sealant belongs on the static joint between frame and wall, usually the internal reveal, and only when that face stays dry enough to paint. It does not replace a brush seal, and an internal-only cartridge does not seal the weather face. The listings are on the acrylic sealant comparison. A purchase through our links can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Colour | Frame duty | Water line | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Everbuild Sika Caulk Once white, pack of 2 | White | Painted internal reveal | Water resistant | 9.2 | about £5.38 | View offer |
 Everbuild Everbuild Everflex 175 acrylic sealant, white, 300 ml | White | Internal and external | Waterproof on the listing | 8.2 | about £6.20 | View offer | |
 151 151 frame sealant, brown, 280 ml | Brown | Internal frames only | Not water resistant | 7.4 | about £3.76 | View offer |
On this page
Three gaps, three answers
The moving gap is a sliding sash, a casement seal or the meeting rail. Historic England points to brush and wiper seals, and to BS 7386 for draught strips. Caulk there glues the window or tears the first time you open it. The internal reveal, a still joint you will paint, is the acrylic job. The weather face is a different joint. Bond It's uPVC guide puts a paintable frame sealant inside and a low-modulus neutral silicone or a hybrid outside. Acrylic is not the default external seal. The wet-face choice is on acrylic sealant versus silicone.
Which acrylic in this set
For a dark internal frame, the 151 brown is the colour match: one 280 ml cartridge at £3.76, 4.3 from 859 reviews, internal uPVC, wood and metal only, and not water resistant. Do not use it outside because the colour looks right. Everflex 175 says internal and external, one white 300 ml at £6.20, with plus or minus 7.5 percent movement. The sheet excludes continuous immersion. Caulk Once is paintable in about an hour. Its page states F 12.5P indoors and out, but these Amazon bullets do not say outside, so use it on an internal reveal. Soudal's 10 mm decorator cap is not a new-window tolerance.
Bead size and weather
If the perimeter gap is deeper than it is wide, push in a closed-cell backer so the acrylic bonds to the frame and the wall, not the back of the chase. Trades mention a backer from about 6 mm to about 12 mm. Those are rules of thumb. Do not plug a 15 mm hole with one squeeze. Gun only in dry weather. Below about 4 °C to 5 °C, wait. Caulk Once's full cure is 3 to 5 days, with paint after a skin in about an hour. Rain on an external bead during that cure is the usual failure. An internal-only tube, including the 151, fails on that face even if the afternoon stays dry. Buy a second tube for a whole house of reveals.
When to stop and call a joiner
Stop if the timber is soft or the sash will not stay up. A bead over rotten wood traps the damp. Listed windows need reversible brush and wiper seals, not a permanent mastic. Stop if the only gap is the closing face of a sash. Replacing silicone with internal brown acrylic is how the weather face opens in the first rain. Internal reveal you will paint: acrylic, brown if the 151 limit fits. Outside: only with an exterior marking, in the dry. Moving sash: no bead.
Common questions
Where on a window is acrylic the right bead?
Acrylic is for the still gap you will paint, most often the internal reveal between frame and plaster. It is the wrong bead on a sliding sash and, unless the cartridge is exterior-rated, on the weather face. Caulk Once suits that painted reveal. The 151 brown suits a dark internal frame. Everflex 175 is the listing here that states internal and external.
Can you caulk a sash so it still opens?
No. A continuous bead on the moving perimeter glues the sash or tears the first time you slide it. Historic England describes brush and wiper seals for that gap. A mastic line at a fixed head or cill is not a bead on the faces that travel. If the sash already sticks, a joiner should see the cords before you add sealant.
Inside the reveal or outside the frame?
Inside, on a dry reveal you will paint, acrylic is the usual choice, including an F-INT product. Outside, use a cartridge marked external or F-EXT-INT, and keep rain off it while it cures. If yours is internal only, like the 151 brown, leave the weather face alone. Many installers prefer a low-modulus silicone or a hybrid on the structural perimeter. Caulk Once's page allows façade use indoors and out. These Amazon bullets do not say outside.
Is brown frame sealant for outside wood-effect frames?
Not this one. The 151 brown title is internal use only, for uPVC, wood and metal, and the listing says it is not water resistant. The colour is for a dark frame you can see indoors. Putting it outside because the foil looks wood-effect leaves the joint open to rain. An exterior frame needs an exterior-rated sealant, or a paintable exterior cartridge you then coat.
How wide a perimeter gap before you need a backer?
Use a backer once the joint is deeper than it is wide, or wider than the cartridge is meant to fill in one pass. The Soudal 10 mm cap is a decorating limit, not a new-window tolerance. Trades mention a backing strip from about 6 mm and a backer from about 12 mm. The bead should wet the frame and the wall only. If the gap is structural, ask what was specified.
Will the bead stop draughts on its own?
It stops a draught through the still gap you have filled. It does not stop draughts at an opening sash, which needs a brush or wiper, and it does not fix a frame whose fixings have failed. If the room is still cold at the glass, the unit or the sash fit is the next question. Call a joiner for rotten timber, a sash that needs re-hanging, or a listed window.
Related pages
Sources
Public UK sources used for this comparison. amazon.co.uk prices, stars and review counts were taken from the listings on 19 October 2026.
- Soudal Decorators Caulk technical data sheet (retrieved 19 October 2026)
- Soudal Decorators Caulk product page (retrieved 19 October 2026)
- Sika Caulk Once (retrieved 19 October 2026)
- Everflex 175 acrylic sealant data sheet (retrieved 19 October 2026)
- Soudal UKCA markings explained (retrieved 19 October 2026)
- 151 brown frame sealant safety data sheet (retrieved 19 October 2026)
- Acle Parish Council draught-proofing note (retrieved 19 October 2026)
- amazon.co.uk listings for the five ASINs in this set (retrieved 19 October 2026)