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Repairing Plaster Walls with Lime Putty

cptbuilders Refurb And Upgrade

Mix Ratios, Scratch Coat Techniques, and Humidity Control

Preserving Cape Town’s Heritage Homes Without Modern Cracks (Or Costly Mistakes)

In 2022, we restored crumbling plaster walls in a Victorian-era Bo-Kaap home, only to watch the repair crack within months. The issue? The homeowner used modern gypsum plaster on lime-based walls—a mismatch that Cape Town’s humidity punished. Heritage homes (think Tamboerskloof or De Waterkant) demand lime putty for a reason. Here’s how to master it, compliant with SANS 0109 historical building standards.

Tools & Materials You’ll Need

  • Lime putty (aged 6+ months)
  • Horsehair or sisal fibers (for tensile strength)
  • Wooden hawk and stainless steel trowel (avoid plastic—it drags)
  • Misting spray bottle (for humidity control)
  • Hydrated lime (for scratch coats—SABS-approved)
  • Sharp sand (0.5–1mm grain size, washed Salt River sand works best)

Step 1: Mix Ratios by Weight, Not Volume

Lime putty is unforgiving. For a base coat:

  • 1 part lime putty : 3 parts sharp sand : 0.5 parts fibers
    Mix for 15 minutes—until it “sings” (a sticky, rope-like consistency).
Mixing lime putty with Salt River sand for a Bo-Kaap heritage home repair.

Cape Town quirk: Tap water’s high mineral content alters curing. Use filtered water from suppliers.

Step 2: Apply the Scratch Coat Like a 19th-Century Pro

  1. Wet the wall: Mist until damp (not soaked).
  2. Throw the mix: Use a flicking motion to embed plaster into cracks.
  3. Scratch lines: Drag a Devil float horizontally for 3mm-deep grooves.

Pro tip: Add 5% pozzolan to accelerate curing in winter.

Step 3: Humidity Control is Non-Negotiable

Lime needs 70–80% humidity to cure without cracking. In dry Karoo-influenced areas like Bellville:

  • Cover walls with hessian sacks (spritz daily).
  • Avoid direct sunlight—work early mornings or late afternoons.
Applying scratch coat with Devil float in a Tamboerskloof renovation

Cape Town horror story: A Constantia homeowner used fans to speed drying… and watched their R30k repair crumble.

Step 4: Finish with a Fine Lime Wash

Finishing lime plaster with traditional wash in a De Waterkant home

After 28 days curing:

  1. Mix lime wash: 1 part putty to 2 parts water. Add iron oxide pigment for Cape Dutch ochre tones.
  2. Apply with a goat-hair brush.
  3. Seal with linseed oil (boiled, not raw).

Common Mistakes (And How to Fix Them)

  • Mistake: Using cement in the mix.
    Fix: Cement traps moisture—heritage walls rot. Stick to pure lime.
  • Mistake: Skipping fibers.
    Fix: Without horsehair, plaster shrinks. We’ve salvaged walls in Sea Point by retrofitting sisal.

When to Call a Pro

If walls have structural cracks (>5mm wide) or asbestos underlayers (common in Maitland’s industrial conversions), hire a SAHRA-certified restorer. Unapproved alterations risk fines up to R100k.

FAQ: Cape Town Edition

Q: Can I repair modern drywall with lime putty?
A: No—modern boards lack breathability. Use lime only on original plaster or brick.

Q: How do I match historic textures?
A: Practice “knobbing” (dabbing with a sponge) or “pebbledash” (throwing sand). 

Need Help?
CPT Builders specializes in SANS 0109-compliant heritage restorations. Contact us for period-accurate repairs!

CPT Builders
8 The Oval Pinelands
Cape Town
7405
021 012 5370