Why is there a ridge where the new plaster meets the old?

Raking light finds a ridge from three metres away in a way that no amount of standing directly in front of it will. It is the most common visible fault in a patched wall, and it comes from a mismatch of levels that is easier to prevent than to correct.

What you'll need

• a wide taping knife — 250mm or wider
• fine surface filler or jointing compound
• a sanding pole or block with a fine abrasive
• a work light to rake across the wall
• a long straight edge (optional)

Step by step

1. Find the true shape of the problem with raking light.Put a lamp or torch close to the wall, shining along it. Mark the high and low areas with a pencil. What looks like one ridge is often a step on one side and a hollow on the other.
2. Check it with a straight edge.Lay a long straight edge across the patch. This tells you whether the new work is proud, whether the old wall is hollow, or both — and they need different treatment.
3. If the new work is proud, sand the step down first.There is no point feathering over a hard step; take it back level with a sanding block or a pole sander using a fine abrasive, with dust control.
4. If the surrounding wall is hollow, fill the hollow rather than the step.Skim a very thin coat over the surrounding area to bring it up, working outwards from the patch.
5. Feather much wider than the patch.The eye detects a change of level over a short distance. Spread the same height difference over half a metre and it becomes invisible. Use the widest knife you have and apply a coat that is thick at the middle and nothing at the edges.
6. Work in several very thin coats, not one thick one.Thick coats shrink, crack and leave their own edges. Two or three coats, each wider than the last, is the standard method and the reason taping knives come in increasing sizes.
7. Sand with a pole or a wide block, not fingertips.Sanding with fingers behind the paper cuts dishes into the surface. A rigid backing spans the area and takes down the high spots only.
8. Check with raking light between every coat.It is the only honest feedback. Judging a wall from directly in front of it in flat light tells you nothing.
9. Prime the repair before painting.New filler and plaster are far more absorbent than the surrounding painted wall, so an unprimed repair reads as a dull patch through the topcoat even when the level is perfect.

Tips

• The wider your knife, the better this job goes. A 100mm filling knife cannot feather; a 250mm or 300mm taping knife can.
• Hold the light at the wall yourself while someone else looks from across the room — the two of you see completely different things.
• Do the final check after the mist coat rather than after the filler. Paint reveals ridges the bare surface hides.
• If a patch keeps showing no matter what, consider skimming the whole wall. It is a bigger job that ends the argument.

Common mistakes

• Feathering only to the edge of the patch — It concentrates the whole change of level into a few centimetres, which is exactly the sharpest transition the eye can detect. Spreading the same difference over a much wider band makes the same amount of material invisible.
• Building the level up in one thick coat — Thick coats shrink as they dry, so the middle sinks and the edges stay proud, and they crack. The repair then needs redoing from a worse starting point.
• Sanding with the paper held in the hand — Fingers press harder than the paper's backing, so each fingertip cuts a shallow dish. Under paint those dishes show as a mottled patch even though the ridge has gone.

If it doesn't work

Questions people ask