Introduction

If you work in older homes, you know plaster‑and‑lath can bite. Hit a hidden cable, pull a loose key, miss the bonding, and you’re back next week fixing a bubble or crack. This guide lays out practical, job‑proven steps to open, patch, and finish plaster‑and‑lath so it blends cleanly with the old work and stays put. We’ll cover assessment, dust, backing, mixes, finishing, and how to set client expectations so you’re not firefighting later.

Table of Contents

• Key Takeaways
• Assess Before You Cut
• Control Dust Like A Pro
• Build A Solid Base: Backing And Key
• Bonding, Mixes, And Build‑Up
• Finish For Invisible Repairs
• Scheduling And Client Expectations
• Frequently Asked Questions
• Conclusion

Key Takeaways

• In general, pre‑cut scanning and test holes reduce chances of hitting hidden services by around half.
• HEPA extraction and light negative pressure commonly cut airborne dust by 70–90% on interior repairs.
• Most callbacks on old plaster trace back to poor backing or bonding; get the substrate right, finishes behave.
• Setting compounds (commonly 20–90 min set) can save a return visit or shorten total programme by a day.
• Clear proposal notes on scope, blends, and paint readiness commonly halve back‑and‑forth with clients.

Assess Before You Cut

Old walls hide surprises: live cables, shallow copper, brittle keys. Many contractors find that the time invested here saves hours later.

Problem

You open up blindly, snag a pipe or cable, and the patch scope doubles. Lath can delaminate if you pry the wrong way.

Solution

• Scan first: use a multi‑scanner for studs/metal/AC cables. In general, doing this reduces service strikes by about half.
• Pilot inspection: cut a neat 30–50 mm inspection slot along lath lines to peek for services and keying.
• Square the opening: cut back to sound plaster with firm keys. Avoid feather‑thin edges—leave a clean arris.
• Protect services: if a cable or pipe crosses, add a timber batten guard before closing.

Example

Small socket relocation leaves a 300 x 200 mm void. You scan, find a diagonal cable, then open a 40 mm slot to confirm. You trim back to solid plaster, notch a batten to sit over the cable for protection, and proceed knowing your fixings won’t bite.

Control Dust Like A Pro

Dust is the number one complaint in lived‑in homes. It’s common for callbacks to start as “the work is fine, but the house was dusty.”

Problem

Grinding, raking and sanding spread fine dust through the property, hurting client satisfaction and triggering clean‑up losses.

Solution

• Room isolation: zip‑door plastic or a tight polythene door, plus floor and soft‑furnishing protection.
• Extraction at source: multi‑tool with vacuum port and HEPA vac. In general, HEPA plus negative pressure cuts airborne dust by 70–90%.
• Air management: place a fan in a window to pull air out; crack a door for make‑up air so dust moves away from the home.
• Wet scrape: lightly mist brittle edges; it reduces dust and avoids tearing keys.

Example

In a lounge, you set a temporary door, cover floors, place a fan in the sash window, and connect your multi‑tool to a HEPA vac. The client stays comfortable, and your end‑of‑day clean is 15 minutes, not an hour.

Build A Solid Base: Backing And Key

If the patch fails, it’s usually because the base was weak, not the finish.

Problem

Hollow patches, drummy sounds, or cracks reappear. Old lath can be broken, missing, or too springy to carry plaster.

Solution

• Backing boards: fix 9–12 mm plywood or plasterboard offcuts behind the hole with screws into studs or with toggle battens. The patch needs firm bearing.
• Reinforcement: where lath is missing or crumbly, fix expanded metal lath (EML) or plasterboard patch and tie it back mechanically.
• Edge support: undercut the surrounding plaster slightly and embed mesh that bridges old to new.
• Fasteners: use coarse‑thread drywall screws or wood screws; keep heads just below the face without crushing.

Example

A 400 x 400 mm hole with missing lath: you add two 25 x 50 mm battens as toggles, screw in a 12 mm plasterboard infill flush with the plaster, then mesh the perimeter. The base is rigid, so coats won’t crack as it cures.

Bonding, Mixes, And Build‑Up

Plaster‑over‑plaster needs chemistry and technique that suit the old substrate.

Problem

New materials don’t bond to dusty, high‑suction, or alkaline surfaces. Skims sheer off or craze.

Solution

• Prime/bond: degrease; vacuum; apply bonding agent (e.g., PVA or proprietary acrylic) as per manufacturer. On high suction, mist water then bond.
• Choose the system:

• Layering: for deep repairs, scratch coat to 8–10 mm, score, allow set; brown coat to near flush; finish skim 2–3 mm and feather.
• Temperature/humidity: aim 10–20°C and moderate humidity. In general, this reduces cracking risk and keeps set times predictable.

Example

Kitchen patch 12 mm deep: you mist, bond, then use a 45‑min setting compound as a scratch, score it, then a second 45‑min build to within 1–2 mm. Finish with a tight skim, feathered 300 mm past the joint. You’re ready to prime the next day.

Finish For Invisible Repairs

Your goal is “can’t see it in daylight.” That takes sequencing and detail.

Problem

Telegraphing edges, texture mismatch, flashing under paint, or stains bleeding through.

Solution

• Feather wide: extend the skim 200–300 mm beyond the patch so the eye can’t find the edge.
• De‑nib, not over‑sand: use light de‑nibbing with a vacuum sander; heavy sanding lops edges and reveals joints.
• Prime smart: alkali‑resisting or stain‑blocking primer if the old wall shows nicotine, water marks, or soot. In general, this prevents bleed‑through that often appears days later.
• Texture match: if walls aren’t dead flat, replicate orange peel, brush marks, or slight trowel texture before painting.

Example

A hallway with prior water staining: you spot prime with stain blocker, skim and feather, de‑nib, full‑prime the wall, then apply two finish coats. Under side‑light, the patch disappears.

Scheduling And Client Expectations

Clear expectations prevent most “quality” disputes that are really timing or scope issues.

Problem

Clients expect one‑visit magic; plaster wants time. Miss this conversation and you inherit moisture and paint issues.

Solution

• Explain the timeline: open/protect/patch (day 1); prime and paint (day 2 or 3) depending on set/dry.
• Paint readiness: in general, skims need 12–24 hours before priming; stain block can add drying time.
• What’s included: agree on paint colour, wall area to repaint (entire wall vs. blend), and who paints.
• Lock it in: many contractors report that capturing these assumptions on site cuts follow‑up questions by half.

Example (With Donizo)

On site, you speak the scope into Donizo: “Square patch approx. 400 by 400, backing board, two‑coat set, skim and feather to 300 mm, stain‑block prime, client to paint full wall.” Photos attached. You send the branded PDF proposal before you leave, the client e‑signs that evening, and you convert to invoice after acceptance. Contractors often report saving 1–2 hours of admin per small patch using this voice‑to‑proposal flow.

Frequently Asked Questions

How Do I Find Services In Old Plaster Walls?

Use a multi‑scanner capable of detecting AC, metal and studs. Cut a small inspection slot along the lath direction to confirm. In general, this two‑step approach halves the chance of striking a cable or pipe. Where in doubt, isolate power and use non‑metallic fixings near suspected routes.

Do I Need A Bonding Agent Over Old Lime Plaster?

Usually, yes. Clean, de‑dust and apply a compatible bonding agent, especially on high‑suction or chalky surfaces. Without it, new gypsum can craze or shear. On deep patches, consider a base coat formulated to bridge old lime to new finish.

Why Do Hairline Cracks Appear After A “Perfect” Skim?

Common causes are movement from weak backing, over‑sanded edges, or fast drying from heat. Strengthen the base (backing board/EML), mesh the perimeter, control room climate, and avoid baking the wall with heaters. A light de‑nib and repaint won’t fix a moving base—rebuild the substrate.

How Soon Can I Paint A Fresh Patch?

In general, allow 12–24 hours for a thin gypsum skim to dry before priming, longer in cool/damp rooms. If stains exist, spot prime with stain blocker and let it cure fully before topcoats. Rushing paint often leads to flashing or blistering.

What’s The Best Way To Price Small Patches?

Define the variables: access, dust protection, depth of repair, paint readiness, and whether you’re painting the entire wall. Many contractors use a minimum visit rate plus add‑ons for deep build‑ups and full‑wall repainting. Capture assumptions in writing to avoid scope creep.

Conclusion

Old plaster‑and‑lath repairs are won in the prep: locate services, control dust, build a rigid base, bond properly, and feather wide. Manage the schedule and the conversation, and the finish will follow. If you want to wrap the admin quickly, capture the scope on site with voice, photos and text in Donizo, send a branded PDF for e‑signature, and convert the accepted proposal to an invoice in one click. Less typing, fewer surprises, and a patch that stays invisible.