How do I repair a cracked or lifting window board?

The internal window board catches everything that runs off the glass. On an MDF board that means the front edge and the corners swell and the paint lifts, and no amount of filler survives if the water keeps arriving.

What you'll need

• Two-part wood filler
• A sharp chisel and a multi-tool
• Replacement board, if needed
• Grab adhesive and packers
• Primer and paint
• Decorator's caulk

Step by step

1. Establish where the water is coming from.Condensation running off the glass is the usual answer. A leak, blocked drains, or a failed external seal are the others, and they need fixing before any repair.
2. Check what the board is made of.MDF swells and delaminates and cannot be dried out. Softwood and hardwood boards can usually be sanded, filled and repainted. Stone or tiled cills are a different job entirely.
3. Repair small damage with two-part filler.Cut back the loose material, apply two-part polyester filler, and sand flat once cured. It is far more water-resistant than decorator's filler and takes an edge better.
4. Decide honestly about replacement.A board that is swollen along the front edge, soft to a fingernail, or lifting at the surface, cannot be repaired durably. Replacing it is a two-hour job.
5. Cut the old board free.Score the caulk lines at the wall and the frame, then cut the board across near each end with a multi-tool and lever the middle out first.
6. Check what it was sitting on.Boards sit on the cavity closer, on brick, or on timber bearers. Note the packing so the new board sits at the same height and the same projection.
7. Template the new board.Card or hardboard, scribed to the reveal at each end. Window reveals are rarely parallel and a straight-cut board leaves a wedge gap at one end.
8. Cut the horns.The nibs that project past the reveal into the room at each end. Mark them from the original board, or from the plaster line, before cutting the ends.
9. Bed and pack the new board.Grab adhesive on the bearing surface, packers to get it level, and weight on it while it sets. It should be dead level or fall very slightly towards the room.
10. Seal the back edge to the frame.A neat bead of caulk or silicone where the board meets the window frame. That joint is where the condensation runs, and an open joint feeds water straight into the board.
11. Prime the edges before painting.MDF end grain and cut edges absorb enormously. Two coats of primer on the cut edges before the topcoat, or the paint will sink and the edge will swell again.

Tips

• Solid timber or a moisture-resistant MDF board is worth the small extra where condensation is a recurring problem.
• Seal every cut edge of MDF, including the underside at the front, before fitting. It is the single thing that decides how long the board lasts.
• If you are replacing the board because of condensation, deal with the condensation as well or you will be doing it again.

Common mistakes

• Filling and painting a swollen MDF board — MDF that has taken up water has expanded permanently and the fibres have lost their bond. Filler sits on a soft base, the paint lifts again within a season, and the board keeps degrading underneath.
• Leaving the joint between board and frame open — That gap is directly under where condensation runs off the glass. Water goes straight into the board's back edge, which is usually unsealed, and the board fails from the inside out.

If it doesn't work

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