How do I patch the plaster over a chased-in cable or pipe?
Filling a deep narrow channel without it sinking, cracking or telegraphing — building up in coats, bridging the line, and finishing wide.
- Difficulty
- intermediate
- Time
- 1 hr 30 min
- Read
- 5 min
- Safety
- warning
Short answer
Fill the depth in stages with a backing material, never in one go, then bridge the whole line with scrim or paper tape, then finish with two or three progressively wider thin coats. A chase filled in one pass sinks as it dries and cracks along its length, and the line then shows through paint forever.
A chase is a long, narrow, deep hole in a wall that is going to move slightly differently from the wall on either side of it. Those three properties — narrow, deep, and a line — are exactly the conditions that produce sunken, cracked, visible repairs, so the method is designed around all three.
Safety
What you'll need
- bonding coat or deep-fill filler
- finish plaster or fine surface filler
- scrim or paper tape
- filling knife and a wide taping knife
- a dusting brush and a vacuum
- primer for the repair
Step by step
- Confirm the cable or pipe is properly installed and protected first.Cables in a chase need mechanical protection or to be installed in a way the wiring regulations accept, and their route matters for anyone who drills the wall later. This is the moment to get it right, because it disappears afterwards.
- Clean the chase out thoroughly.Brush and vacuum. Loose dust in the bottom of the chase is a bond breaker, and dust is exactly what a chase collects.
- Dampen a very absorbent background, or prime it.Brick and block will suck the water out of the first coat and leave it powdery. Controlling that suction is what makes the first coat bond.
- Fill the depth with a backing material, in layers.Bonding coat or a deep-fill filler, built up in the thicknesses the product allows, letting each layer set. This is the step that prevents shrinkage cracking.
- Stop the backing a few millimetres below the wall surface.Leave room for tape and for the finishing coats. Filling the chase flush leaves you nowhere to go and forces the finish coat to sit proud.
- Bridge the whole length with tape.Scrim or paper tape bedded down the line, over the joint between new material and old wall on both sides. The tape is what stops a crack running down the line as the two materials move.
- Finish in two or three progressively wider coats.Thin coats, each wider than the last, each feathered to nothing. A chase is a long line, so the finished repair should be a wide gentle band, not a narrow strip.
- Sand with a wide block or a pole sander.Long straight repairs show every dish and hollow. A rigid wide backing keeps the surface flat along the length.
- Prime the repair before painting.Fresh plaster and filler drink paint. Without priming, the whole line reads as a dull stripe through the topcoat — a fault that is entirely about absorbency rather than colour.
- Record where it is.Photograph the chase before you fill it, with a tape measure in shot. In five years, when somebody wants to put a shelf there, it is the only record that exists.
Tips
- Cables run vertically and horizontally from accessories in defined safe zones. Keeping to those zones when chasing means the next person has a fighting chance of not drilling into it.
- If the chase is very deep, a mesh or scrim laid into the backing coat as well as over the finish helps considerably.
- Feather the finishing coat much wider than the chase — a 25mm chase can easily justify a 300mm wide finished band.
- In a plasterboard wall, the equivalent job is different: the cable runs in the void and the repair is a board patch, not a filled chase.
Common mistakes
- Filling the whole depth in one pass — Deep filler shrinks as it cures. A single deep fill sinks below the surface along its length and cracks, so the repair shows as a sunken line the moment the wall is painted. Layers, each within the product's stated thickness, do not.
- Skipping the tape — The join between the fill and the original wall runs the whole length of the chase, and the two materials shrink and move at different rates. Without tape bridging it, a hairline crack appears along one or both edges, usually within the first heating season.
- Not priming before painting — The repair is far more absorbent than the surrounding decorated wall. The topcoat sinks in over the repair and dries to a different sheen, so a perfectly flat repair still shows as a stripe.
If it doesn't work
A hairline crack down one edge of the repair
Cause: No tape across the junction — Fix: Rake the crack open, tape across the full length, and finish over in thin wide coats.
The line has sunk below the wall surface
Cause: Filled too deep in one pass and shrunk — Fix: Fill the hollow in thin coats, feathered wide, checking with a straight edge and raking light between coats.
A visible dull stripe after painting
Cause: Repair not primed — Fix: Prime the repair line, let it dry, then repaint the wall corner to corner rather than spot-painting.
Questions people ask
Can I use ordinary filler for the whole depth?
Not sensibly. Lightweight fillers are designed for shallow work and shrink noticeably in depth. Use a bonding coat or a filler specifically sold for deep fills for the body of the chase, and keep the fine filler for the last couple of millimetres.
Should I use scrim or paper tape?
Either works here. Scrim is self-adhesive and faster and is fine under a plaster or compound finish; paper tape is thinner and gives a flatter finish but has to be bedded in compound. On a long straight chase, most people find scrim easier to keep straight.
Do I need to plaster it, or will filler do?
Filler is perfectly acceptable for a domestic chase, provided the depth is built up in layers and the line is taped. Plaster is stronger and gives a better surface over a large area, but the technique — layers, tape, wide feathering — matters far more than the choice of material.