Plaster and Rendering

Part of Lime & Cement

Plastering walls with lime-based coats for protection, insulation, and finish.

Why This Matters

Bare stone and brick walls are porous, rough, and vulnerable to weather. Lime plaster and render transform these raw surfaces into smooth, weather-resistant, and visually clean walls that last centuries. Render (external coating) sheds rain and protects masonry from freeze-thaw damage. Interior plaster creates smooth surfaces for living and working, improves light reflection, and provides a degree of air-sealing that improves thermal comfort.

Lime plaster is also inherently antimicrobial — its high alkalinity inhibits mold and bacterial growth, making plastered buildings significantly healthier than uncoated ones. In a rebuilding scenario, this is not a cosmetic luxury but a genuine public health measure. Plastering also seals gaps that harbor insects and vermin.

The techniques described here require only lime putty, sand, water, basic hand tools, and patience. No modern materials or equipment are needed.

Materials Preparation

Lime Putty

Use well-matured lime putty, ideally aged at least three months. For fine finish plasters, putty aged one year or more produces noticeably smoother results. The putty should be smooth, free of lumps, and have a consistency like thick cream cheese.

Sand and Aggregates

Coat	Sand Type	Maximum Grain Size
Scratch coat (first)	Coarse, sharp sand	4-5mm
Brown coat (second)	Medium sharp sand	2-3mm
Finish coat (third)	Fine sand or marble dust	1mm or less

Sharp, angular sand is essential — it locks the plaster together mechanically. Rounded sand produces weak, crumbly plaster. Wash sand to remove clay and organic matter if necessary.

Fiber Reinforcement

Adding fiber to base coats dramatically reduces cracking:

• Animal hair — Cattle, horse, or goat hair. Cut to 25-50mm lengths. The traditional standard.
• Plant fiber — Chopped straw, hemp fiber, sisal, or jute. Cut short to avoid lumps.
• Quantity — Add approximately one handful of fiber per bucket of mixed plaster. Mix thoroughly to distribute evenly.

Preparing Hair

Animal hair must be clean and free of grease. Wash in hot water with wood ash (lye), rinse, and dry before adding to plaster. Greasy hair won’t bond with lime.

Mix Proportions

Coat	Lime : Sand Ratio	Fiber	Thickness
Scratch coat	1 : 3	Yes — generous	10-15mm
Brown coat	1 : 2.5	Optional — light	8-12mm
Finish coat	1 : 1.5 (or pure putty + marble dust)	None	2-5mm

Surface Preparation

For Stone Walls

1. Rake out joints — Remove loose mortar to a depth of 10-15mm. This gives the plaster something to grip.
2. Brush clean — Remove dust, loose material, and organic growth with a stiff brush.
3. Dampen thoroughly — Spray or brush water onto the wall until the stone is saturated but not dripping. Dry stone sucks moisture from plaster too rapidly, causing it to crack and debond.

For Brick Walls

1. Rake joints — Same as stone, 10-15mm depth.
2. Dampen — Bricks are highly absorbent. Multiple wettings may be needed, especially in warm weather.

For Timber-Framed Walls (Wattle and Daub Panels)

1. Install lath — Split wooden strips (riven lath) nailed horizontally across studs with 8-10mm gaps between them. The plaster squeezes through the gaps and curls behind the lath, forming mechanical keys.
2. Alternative: woven lath — Weave flexible sticks between upright stakes for a wattle substrate. The rough surface provides excellent grip.
3. Dampen lath — Wet wooden lath before plastering to prevent rapid moisture absorption.

Application: The Three-Coat System

First Coat: Scratch Coat (Arriccio)

The scratch coat is the foundation. Its job is to grip the wall surface and provide a flat, even base for subsequent coats.

1. Load the hawk — Place a mound of mixed plaster on a hawk (flat board with handle underneath) held in your non-dominant hand.
2. Apply with trowel — Scoop plaster from the hawk and press firmly onto the wall surface, working upward from the bottom.
3. Press hard — The first coat must be forced into joints, gaps, and lath spaces. Don’t just smear it on — drive it into the substrate.
4. Build to thickness — Apply 10-15mm in a single pass. Don’t try to make it smooth.
5. Scratch the surface — While still soft (within 2-3 hours), drag a notched tool, comb, or fork across the entire surface to create horizontal grooves 3-5mm deep. These scratches provide mechanical key for the next coat.
6. Cure — Keep damp for at least 3-5 days. Mist with water morning and evening. Do not apply the second coat until the scratch coat is firm to the touch.

Timing Between Coats

Wait a minimum of 7 days between coats — ideally 2-3 weeks. Each coat must begin carbonating before the next is applied. Rushing produces delamination (coats separating from each other).

Second Coat: Brown Coat (Corpo)

The brown coat levels the wall and provides the true flat surface.

1. Dampen the scratch coat — Spray water until the surface is damp but not wet.
2. Apply plaster — Work from bottom to top, building to 8-12mm thickness.
3. Straighten with a darby — A darby is a long, flat straightedge (1-1.5m). Drag it upward in a sawing motion across the wet plaster. This flattens high spots and reveals low spots.
4. Fill low spots — Add plaster to any hollows and re-straighten.
5. Float the surface — Once reasonably flat, rub with a wooden float (flat board with handle) in circular motions. This brings fine material to the surface and compacts the plaster.
6. Texture for key — Leave the surface with a slightly rough texture to accept the finish coat. Do not polish smooth.
7. Cure — Keep damp for 5-7 days minimum. Wait at least 7-14 days before applying the finish coat.

Third Coat: Finish Coat (Intonaco)

The finish coat is the visible surface. This is where skill shows.

1. Dampen the brown coat — Light misting only.
2. Apply very thin — Only 2-3mm thick. Use a steel trowel or very smooth wooden trowel.
3. Work in sections — Complete one area at a time, wet edge to wet edge. Joints between dried sections will always show.
4. Compress and polish — As the plaster begins to firm (within 30-60 minutes), press hard with the flat of the trowel, polishing the surface. Multiple passes at increasing firmness produce an increasingly smooth finish.
5. Final burnishing — For a polished finish (marmorino), continue troweling as the plaster sets. The heat and pressure from troweling realign calcium hydroxide crystals at the surface, creating a dense, slightly shiny finish that becomes waterproof as it carbonates.

Exterior Rendering

Exterior render faces rain, frost, wind, and sun. Techniques differ from interior plaster:

Key Differences from Interior Work

• Use hydraulic lime if available — it sets harder and resists water better
• Limit to two coats — A thick render is more likely to crack. 15-20mm total is sufficient.
• Keep the surface rough — A floated (wood float) finish is preferred over a troweled finish. The texture sheds water better than a smooth surface, which can trap water films.
• Add pozzolanic material — Crushed brick, volcanic ash, or fired clay powder mixed into the plaster improves water resistance.

Rendering Procedure

1. Prepare and dampen the wall as for interior work
2. Apply scratch coat (10-12mm) — use coarser sand and generous fiber reinforcement
3. Scratch deeply — 5mm grooves minimum
4. Wait minimum 2 weeks — exterior coats need longer between applications
5. Apply finish coat (8-10mm) — use a wooden float to leave a textured surface
6. Protect from rain — Cover fresh render with tarps or boards for at least one week. Rain on uncured lime render washes it away.

Bellcasting and Drip Details

Water running down a wall face concentrates at ledges and sills, causing damage. Prevent this:

• Bellcast — Flare the render outward at the base of the wall so water drips clear of the foundation
• Drip grooves — Cut a groove on the underside of any projecting ledge or windowsill to break the water film

Limewash as Final Coating

After plaster has cured (minimum 4-6 weeks), apply limewash — diluted lime putty brushed on in thin coats.

1. Dilute lime putty with water to milk consistency
2. Apply with a large, soft brush in thin, even coats
3. Work quickly — don’t over-brush, as this pulls the coating off
4. Apply 3-5 coats, allowing each to dry completely (24 hours minimum)
5. Each coat is nearly transparent when wet but dries opaque white

Limewash creates a sacrificial protective layer that can be renewed every few years. It fills hairline cracks and provides UV protection for the plaster beneath. Adding natural pigments (iron oxides, carbon black, ochres) creates colored limewash.

Troubleshooting

Problem	Cause	Prevention/Fix
Cracking (map pattern)	Drying too fast; too much lime	Cure properly; adjust mix ratio
Cracking (linear)	Movement in substrate	Add fiber reinforcement; ensure substrate is stable
Delamination (coat peeling)	Previous coat too smooth; insufficient dampening	Scratch between coats; dampen before applying
Hollow areas (sounds hollow when tapped)	Poor adhesion behind plaster	Cut out and re-apply; ensure good key
Powdery surface	Carbonation failure (dried too fast)	Keep damp longer; apply limewash to consolidate
Staining	Salts migrating from substrate	Use salt-free materials; apply limewash barrier

Tools Required

Tool	Purpose	DIY Alternative
Plastering trowel	Applying and smoothing	Flat hardwood board, polished smooth
Hawk	Holding plaster	Square board with handle underneath
Darby	Straightening	Any straight board 1.2-1.5m long
Wood float	Texturing	Flat board with handle, sanded smooth
Scratch tool	Keying between coats	Fork, notched stick, or comb
Bucket	Mixing	Any large container
Spray bottle	Dampening	Brush dipped in water also works