How do I fill the gap between architrave and the wall?

There are three different joints around an architrave and they want three different products. Treating them all the same is why the gap is back by next Christmas.

What you'll need

• Decorator's caulk and a skeleton gun
• Fine surface filler
• A damp cloth and a bowl of water
• A filling knife
• Fine abrasive paper

Step by step

1. Identify which joint you are filling.The architrave-to-wall joint moves seasonally and needs a flexible caulk. The mitre and the nail holes are within one piece of timber and take a rigid filler that sands flat.
2. Work out why the gap is there.A gap that follows an uneven wall is normal. A gap that has opened recently, or that is wider at one end, may be a frame moving — worth fixing before decorating over it.
3. Fix the architrave first if it is loose.Caulk is not an adhesive. Pin the architrave properly into the frame, and only then fill.
4. Cut the nozzle small.A 3–4mm opening cut at 45 degrees. Almost everyone cuts too much off, which puts three times the caulk you need into the joint.
5. Run a continuous bead.Steady pressure, steady speed, gun held at a consistent angle, moving away from yourself. Stopping and starting shows in the finished line.
6. Tool it with a wet finger.One pass, one direction, wiping the finger between passes. Water is what stops the caulk dragging. Going back over a tooled joint is what makes it look lumpy.
7. Wipe the excess immediately.A damp cloth across the wall and the timber while the caulk is wet. Once it skins, it can only be cut off with a knife.
8. Fill wide gaps in two passes.Anything over about 5mm slumps if filled in one go. Fill most of it, let it skin overnight, then run a finishing bead.
9. Use filler on the mitre and nail holes.Punch the pins below the surface first, fill slightly proud, and sand flush once dry so the joint disappears.
10. Paint promptly.Caulk is not a finish. It picks up dust, discolours and shrinks slightly if left bare, so prime and paint within a few days.

Tips

• Buy a decent skeleton gun with a smooth thrust and a release trigger. Cheap ratchet guns cannot be stopped cleanly and that is most of what makes a bad bead.
• A drop of washing-up liquid in the water bowl makes the finger glide and gives a noticeably cleaner line.
• Caulk shrinks slightly as it dries. On a deep joint, expect to need a second thin pass — plan for it rather than being disappointed.

Common mistakes

• Using powder or ready-mixed filler on the wall joint — That joint opens and closes with humidity and with the building's own movement. Rigid filler has no elasticity, so it cracks along its length, and the crack is more visible than the original gap.
• Cutting the nozzle too big — Too much caulk means a wide, thick bead that shrinks unevenly, smears onto the wall and takes an age to tidy. A small nozzle and a second pass beats a big nozzle every time.

If it doesn't work

Questions people ask