Painting Sash Windows: Order of Work, Cut Lines and Sticking

The commonest painting call we get is not about the finish. It is that the windows were decorated last summer and now two of them will not move at all. Nothing has rotted and nothing has broken. The paint has simply glued the sash to the frame, or filled a running clearance that was never more than a couple of millimetres to begin with.

Painting a sash window is not difficult work, but it is different work from painting a door. A door only has to look right. A sash has to look right and still slide, and almost everything that goes wrong comes from ignoring the second half of that.

Why a freshly painted sash seizes

Two separate mechanisms, and they need different answers.

Film build. A full coat of exterior paint dries to somewhere around three or four hundredths of a millimetre. That sounds like nothing until you count the layers. Ten redecorations over a century is roughly a third of a millimetre on a face, and a sash edge running against a parting bead has two coated surfaces closing on each other, so you have lost the better part of a millimetre from a clearance that was perhaps two. Add a damp week and the timber swells into what is left. This is why old windows get progressively tighter and why stripping back, rather than adding another coat, is sometimes the only honest fix.

Blocking. This is the one that catches people out after a single summer. Paint continues to cure for weeks after it feels dry, and while it cures, two freshly coated surfaces pressed together will bond to one another. Modern water-based acrylics are particularly prone to it. The window feels touch-dry in four hours, gets shut that evening, and by the morning the meeting rails have welded.

Both problems are avoidable, and neither is a paint-quality issue. If your window is already stuck, work out which cause you have before attacking it, because the full range of reasons a sash jams includes several that a scraper will not solve.

Before you sand anything, think about lead

Lead paint was banned from sale to the general public in the UK in 1992, with narrow exceptions for specialist heritage use. On a London period house the outer coats will be modern, but the layers underneath are very likely to contain lead, and the older the window the more certain that becomes.

Lead in a sound, intact film is not doing anyone harm. The hazard is created entirely by how you remove it. Dry scraping, dry sanding, power sanding without extraction and burning off with a flame or a very hot gun all convert a stable coating into dust or fume, and window joinery is exactly where children's hands and open ground below tend to be.

Practical approach on a domestic job:

• Assume lead is present on anything painted before the 1990s. Cheap swab tests exist and are worth the few pounds for peace of mind, though they are indicative rather than definitive.
• Leave sound paint alone. Wash it, abrade it lightly by hand with wet abrasive paper, and overcoat. Encapsulating good paint is safer than removing it and gives a better finish anyway.
• Where paint must come off, use wet methods, a chemical stripper suited to old oil paint, or a low-temperature infrared or steam plate rather than a naked flame. Keep temperatures modest.
• Sheet the ground below, work with the window open and the room ventilated, vacuum with a filtered machine rather than sweeping, and bag the debris.
• Keep children and pregnant women away from the work and clean up thoroughly at the end of each day rather than at the end of the job.

Preparation, in the order that actually matters

Painting is about ninety per cent preparation, and on a sash window the sequence is not arbitrary.

1. Fix the timber first. Paint over soft wood buys you one season. Probe the bottom rail, the cill and the base of the pulley stiles with a bradawl. If it sinks in easily, that is a repair job, not a decorating job, and wet rot in sash windows explains what needs cutting out.
2. Sort the putty before you paint, not after. Loose, cracked or missing putty lets water straight to the rebate. Rake it out and reputty where needed. Fresh linseed oil putty must skin before it can take paint, which in decent weather is a week or two and in a cold damp spell considerably longer. The full method is in how to reputty a sash window, and linseed versus acrylic putty covers the choice.
3. Check moisture. Timber should be genuinely dry before it is coated, and around 18 per cent moisture content is the working ceiling most decorators use. A cheap pin meter settles the argument. Paint applied over damp timber will blister within a year.
4. Abrade, do not obliterate. The aim is a key and a flat surface, not bare wood. Feather back any flaking edges so they do not telegraph through.
5. Knot and prime the same day you expose bare timber. Resinous knots get knotting solution. Bare wood gets primer, and end grain, especially the bottom of the stiles and any spliced joint, gets an extra flood coat because that is where water enters fastest.
6. Fill after priming, not before. Filler on bare wood sucks and shrinks. Prime, fill, spot-prime the filler.
7. Dust off properly. A tack cloth or a damp lint-free rag. Everything you leave on the surface stays in the finish.

The order of work on a box sash

This is the bit most people get wrong. You cannot paint a double-hung window in one position, because half the timber is hidden. The traditional sequence reverses the sashes so every face becomes reachable, and it goes like this.

1. Score the paint line. Run a sharp knife along the joint between the staff bead and the sash, and along the parting bead, before you try to move anything. On a window that has been painted shut this stops you tearing a strip of film off the frame.
2. Free both sashes and check they run. If a sash does not move now, it will not move better with more paint on it. Sort it first.
3. Reverse the sashes. Push the top sash down and raise the bottom sash so the two cross over near the middle of the opening.
4. Paint the top sash first, working on the parts now exposed: the bottom rail of the top sash, the lower part of its stiles, the meeting rail, and the frame above it. Also do the head of the frame and the top of the outer lining while you can reach them.
5. Let it go off, then return the sashes roughly to their normal positions, leaving both open a few inches.
6. Paint the rest of the top sash and then the bottom sash: stiles, rails, glazing bars, and the face of the meeting rail.
7. Paint the frame: the staff bead, the visible face of the pulley stiles, the outer lining, the soffit and the cill. Cills take the most weather, so give them an extra coat.
8. Leave the window open. Both sashes off their stops, for as long as the paint needs. Overnight is not enough for a full gloss system.
9. Run the sashes daily for the first week. Slide them the full travel each morning. That single habit prevents almost all blocking.

Half a day per window is realistic if the preparation is already done. Professionally, external redecoration of a sash window runs somewhere around £150 to £350 in London depending on size, condition and access, and rises sharply once scaffold or a tower is involved. Where putty and timber repairs are needed as well, decoration usually gets folded into a wider overhaul.

What gets paint, and what must not

The bead faces are worth labouring. The parting bead and staff bead are the only things holding the sashes laterally, and they are running surfaces in the same sense that a drawer runner is. Once you have finished painting, rub a plain candle or a block of paraffin wax down those faces and down the sash edges. It costs nothing, it makes the window noticeably lighter to lift, and it stops the last of the tackiness grabbing.

Take the fasteners and lifts off before you start. Two screws each, and the difference between a window that has been decorated and one that has been painted round is visible from the other side of the room.

The cut line onto the glass

Paint must run past the putty and a little way onto the glass. This is not decoration, it is a seal. The joint between the putty face and the glass is the point where rain would otherwise track behind the putty and into the rebate, which is how bottom rails rot from the inside out.

The trade convention is 2mm to 3mm onto the glass beyond the putty line, cut clean and straight. It should look deliberate. A wobbly 6mm line reads as sloppy from the pavement, and a line that stops short of the glass leaves the seal open.

Cut it freehand with a good 25mm angled sash brush if you can. Load the brush lightly, rest the heel of your hand on the glass, and pull the line in one movement per pane rather than dabbing. If freehand is beyond you, low-tack tape set 2mm off the putty works, but pull it while the paint is still soft or it will lift the edge. Scraping the glass afterwards with a blade is the third-best option and tends to break the seal you just made.

Choosing a system

There is no single right answer, and the honest comparison is short.

Traditional oil-based systems (primer, undercoat, gloss) flow out beautifully, give a hard film, and are what most period sashes have on them. They are slower to recoat, they yellow in shade, and reformulation for lower solvent content has made some modern versions less durable than the ones people remember.

Water-based acrylic systems dry fast, stay flexible, hold colour and are far pleasanter to use. Their weakness is exactly the one that matters here: they block. Two coated surfaces in contact while still curing will stick. If you use them on sashes, extend the open time and run the windows daily.

Microporous coatings are widely oversold. They allow some vapour out, which helps on timber that occasionally gets damp, but they will not save wet or rotten wood and they still need sound preparation. Treat the label as a useful property, not a rescue.

Whatever you choose, stay within one manufacturer's system and follow their recoat intervals. Mixing an oil undercoat with an acrylic topcoat, or overcoating before the previous layer has cured, causes more failures than any product on its own. On bare timber expect primer, two undercoats and a finish. On sound existing paint, a light abrade and two finish coats is usually plenty.

Dark colours are worth a thought on a south-facing elevation. A near-black sash in full sun runs much hotter than a white one, which means more movement in the timber and more work for the film.

Weather and timing

Most exterior coatings specify a minimum of around 8°C to 10°C, and the surface needs to be a few degrees above the dew point or the film will bloom. In London that realistically means April to early October for full external work, and it means stopping by mid-afternoon in autumn so the film sets before evening dew lands on it.

Do not paint in direct hot sun either. The surface skins before the body of the coat has flowed, which gives you brush marks and poor adhesion.

What to do next

If the windows still move, this is a weekend job per window and worth doing yourself, provided you take the lead precautions seriously and resist shutting them too soon. Score the line, reverse the sashes, keep the beads bare, wax the runs, and leave them open.

If the sashes are already seized, or the putty is failing, or a bradawl goes into the bottom rail, painting is the last operation rather than the first. That is overhaul territory, and our sash window restoration work normally takes the sashes out, repairs and reputties them, and returns them properly decorated in one visit. Working out where your windows sit on that scale is what a seasonal maintenance check is for, and if you would rather someone else made the call, our sash window repair service covers every London borough.