How do I paint timber windows so they do not stick shut?

A window that sticks after painting has not swollen — the two paint films have fused. Paint remains soft for days after it feels dry, and a closed window presses two soft films together with all the force of the catch.

What you'll need

• Exterior primer, undercoat and topcoat, or a self-undercoating system
• A 25mm and a 50mm brush
• Abrasive paper, 120 and 180 grit
• Filler and knotting solution
• Masking tape or a paint shield
• A candle stub

Step by step

1. Choose the weather.Dry, mild, and with a full day forecast. Painting exterior joinery in the evening or in autumn damp traps moisture under the film and is why so much exterior paint fails early.
2. Deal with the timber first.Rake out failed putty, cut out rot, fill, sand and knot any resinous areas. Paint over a bad substrate fails on schedule regardless of how good the paint is.
3. Sand and dust off properly.120 grit to key the old paint and flatten filler, 180 to finish. Dust with a damp cloth, not a dry one, and let it dry.
4. Paint the opening faces first and thin.The edges that touch when the window is closed. Thin coats here, and only as many as needed for protection — this is where sticking happens.
5. Work in the right order on the face.Glazing bars and rebates, then the rails, then the stiles, then the frame. Working small to large means you tidy the awkward parts while the paint is wet.
6. Keep paint off the running surfaces on a sash.The parting bead and the pulley stile faces should be waxed, not painted. Paint there is the main reason sash windows seize.
7. Take the paint onto the glass deliberately.About 2mm onto the glass over the putty line. That seals the joint and stops water tracking behind the putty. A steady hand beats masking tape here.
8. Wedge the window open while it dries.Open at least 25mm, held with a wedge, for as long as you can. Overnight at minimum, longer in cool weather.
9. Wax the contact faces before the last coat cures.Once the final coat is hard, rub a candle stub along the closing faces. It stops the two films bonding when the window is finally closed.
10. Respect the recoat times.Touch-dry is not recoatable and recoatable is not hard. Read the tin and add time in cool weather; most sticking problems come from closing a window on paint that was only touch-dry.
11. Do not paint the gaskets or the ironmongery.Take the stays and fasteners off, or mask them. Painted hinges bind and painted seals stop sealing.

Tips

• Paint the opening lights in the morning so they have the whole day open before the evening damp arrives.
• A window that is stuck by paint can usually be freed by running a sharp knife along the joint and easing the sash with a broad scraper. Do it before the paint is fully cured and there is much less damage.
• Microporous exterior paints move with the timber and shed water while letting it breathe. On a timber window they last noticeably longer than a traditional oil gloss.

Common mistakes

• Closing the window at the end of the day — Paint that feels dry is still soft for several days. The catch pulls two soft films together and they fuse into one, so the window can only be opened by cutting the joint and damaging the finish.
• Building up thick paint on the closing edges — Every coat adds thickness to both faces, and after a few repaints the accumulated film is thick enough to make the window bind mechanically. Those faces need the fewest, thinnest coats.

If it doesn't work

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