Should I seal the top and bottom edges of a door?

A timber door that sticks in winter and swings free in summer is not badly hung. It is a door that is absorbing and releasing moisture through the surfaces nobody ever finishes. The top edge is invisible, the bottom edge is inaccessible with the door hanging, and the planed edge after a fitting is bare by definition. All three are where the water goes in.

Step by step

1. Check the edges you cannot see.Take the door off, or open it fully and use a mirror and a torch. On most doors the top edge is completely bare timber. On many, so is the bottom. Run a hand over them — bare timber feels dry and slightly rough where a finished surface is smooth.
2. Understand why it matters.End grain absorbs moisture many times faster than a face. A door with two bare ends behaves like a wick: it takes up damp air in autumn, swells across its width and height, and sticks. Sealing every surface makes the whole door move much more slowly and much less.
3. Take the door off to do it properly.The bottom edge cannot be finished with the door hanging. Take it off, lay it across two trestles or stands, and do both ends and both faces in one session. This is the difference between a job that lasts and a smear applied through a gap.
4. Sand the bare edges lightly.A rub with medium then fine abrasive to remove saw marks and open the surface. Dust off thoroughly. End grain drinks finish, so a smooth surface uses less.
5. Prime or seal the end grain first.End grain takes two or three coats to seal properly. Use a primer thinned slightly for the first coat on painted doors, or the manufacturer's recommended base coat for a stained or oiled finish, and let it soak in before the next.
6. Finish to match the faces.Paint, varnish, oil or stain — whatever the faces have. The point is to slow moisture movement equally on every surface, so a door sealed on the faces with paint and on the edges with a different product will still move unevenly.
7. Seal any edge you plane.The moment you trim a door to fit, you have created fresh bare end grain or fresh bare edge. It gets sealed before the door goes back on, every time, even if it is the bottom edge nobody will ever see.
8. Pay particular attention to external doors.On an external timber door the bottom edge is the wettest surface in the whole building. It should be sealed, and it should be checked every year or two, because it is where a door starts to rot.
9. Do the same on the hinge and lock cut-outs.Hinge recesses, the lock mortice and the latch mortice are all bare timber inside. On an external door they are a route in for water. A brush of primer or sealer into each before the hardware goes back costs a minute.
10. Let it cure before rehanging.A door hung while the edges are still soft will pick up marks and, on an external door, will trap solvent. Follow the product's recoat and cure times and be patient at the end.

Tips

• Doors are worth doing at the same time as decorating a room. The door is off anyway and the finish is already open.
• On a new door bought unfinished, seal it before hanging. It is much easier flat on trestles and it stops the door taking up moisture from a freshly plastered room.
• If a door sticks only in one season and the gaps are otherwise even, sealing the edges is a more durable answer than planing it, because a planed door is loose again in summer.

Common mistakes

• Planing a door that sticks in winter — It cures the symptom in December and leaves a door with a visible gap in July. Seal it instead, and only plane if the gap is genuinely wrong in both seasons.
• Finishing the faces and not the edges — The door then absorbs and releases moisture unevenly, which is what makes a door bow as well as swell.
• Using a different product on the edges — Different finishes have very different moisture permeability. Matching the faces is what keeps the movement even.

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