Living in a period property comes with beautiful original features. But older home owners will also know they don’t stay pristine and need proper maintenance. Old lime plaster walls and ceilings are prone to develop cracks, small holes, and hollow sounding patches.
Whilst severe issues mean considering replastering, minor issues can be fixed without calling in a specialist plasterer. If you use correct breathable lime fillers, and the correct repair technique, patch repairs can be both straightforward and effective.
Assessing Plaster Before Repair
Don’t just reach for your filler, check the whole room you’re working in before beginning. Tap gently over all wall surfaces. Listen for hollow sounds, which usually mean blown plaster. This is when the lime plaster has detached from the wall behind. You should also carefully examine crumbling sections to assess which areas have failed, and which are still good.
If you locate old repairs made in cement or gypsum based plasters, removal should be considered. These materials aren’t compatible with lime substrates and can cause cracks. If they aren’t too large, consider removing them to create a clean base for long lasting repair.
Repairing Cracks and Small Holes in Lime Plaster
Cracks are the most common issue in lime plaster. How you deal with them depends on their size:
- Hairline cracks - These are often cosmetic. Fill with a fine lime filler such as Auro 342 High Grade Lime Filler, smoothing flush with the surface.
- Small to medium cracks - Open the crack slightly with a scraper or the edge of a filling knife. This allows the lime filler to key into the plaster properly. Dampen the edges lightly with a fine spray of water before applying filler.
- Larger cracks and small holes - Take a slow and methodical approach, building up the repair in thin layers. Apply the first layer of filler with a flexible filling knife or small trowel, pressing it firmly into place. Allow it to firm up before adding another layer until flush with the wall surface.
Tools You’ll Need
- Filling knife or small plasterer’s trowel for applying filler.
- Scraper for opening cracks and removing loose material.
- Small spray bottle for lightly dampening surfaces.
- Sanding block or fine abrasive paper for smoothing once dry.
Checking the Finish
Once the filler has dried, run your hand over the surface. It should feel smooth and level with the surrounding plaster.
Any proud areas can be gently sanded back with a fine abrasive block. If you find slight depressions, skim a thin layer of filler over the top and smooth again. Repeat the process until you have a finish flush with the surrounding plaster.
Preparing Plaster for Paint
Before painting, make sure the repair is fully dry and dust free. Brush away any loose particles and wipe with a barely damp cloth.
A thin mist coat of breathable paint on fresh plaster can help bind the surface before applying full coats. Always use lime or mineral paints rather than standard emulsions.
How to Patch Repair Blown or Missing Plaster
Where plaster has fallen away or lost adhesion, you’ll need more than a quick crack fill.
Here’s how to tackle small patch repairs yourself:
- Remove all loose or hollow sounding plaster until you reach a firm edge.
- Lightly dampen the area so the wall doesn’t suck moisture out too quickly.
- Apply lime plaster or lime filler in thin coats, pressing each layer well into place.
- Smooth and feather the edges so the repair blends into the surrounding wall.
For small chased-in sections, for example after rewiring, lime fillers are usually enough. For larger missing patches, finishing with a coat of lime plaster will give a stronger and smoother result.
If you uncover wide areas of blown plaster or need to work on ceilings, it’s worth calling in a professional lime plasterer. These jobs can quickly grow beyond a DIY patch.
Finishing With Breathable Paint For Lime Plaster
Once your repairs have cured, you’ll want to decorate. It’s vital to use breathable paints - standard emulsions or vinyl paints will trap moisture and undo your good work.
Choose natural paint finishes which have low Sd values. These coatings allow the wall to breathe, keeping moisture in balance and helping prevent damp from building up.
Why Breathable Materials Keep Homes Healthy
Lime plaster and natural paints are more than just traditional - they perform a vital function for the health of your building fabric. Older houses with solid walls need to regulate moisture, and vapour-open materials like lime plaster make this possible.
The key measure of breathability is the Sd value (steam diffusion-equivalent air layer thickness). A lower Sd value means higher breathability.
For example:
- A highly breathable paint will have an Sd value of less than 0.14 m. Truly breathable paints will have an Sd value of 0.5m or less to be suitable for use in heritage buildings.
- Limewash and lime paints achieve very low Sd values, making them highly vapour permeable.
- Modern vinyl and plastic emulsions usually have Sd values many times higher, meaning they trap moisture inside walls.
By allowing moisture to pass safely through walls rather than becoming trapped, breathable finishes help prevent damp patches, mould growth, and damage to building fabric.
Common Types of Breathable Paint
Different breathable paints each have their own qualities, ingredients, and best uses. Here’s the most common types of natural, breathable paints.
- Lime paint - Made from slaked lime and mineral pigments. Highly breathable with very low Sd values, it is ideal for lime plaster, stone, and other traditional substrates. Best for heritage interiors and exteriors where moisture movement must not be restricted. Not suited to modern gypsum plaster or non-porous surfaces.
- Clay paint - Contains natural clays, chalk, and plant-based binders. It creates a matt, velvety finish and is well suited to interior walls and ceilings. It regulates humidity effectively, making it excellent for living spaces and bedrooms.
- Silicate paint - Based on potassium silicate (water glass). It chemically bonds with mineral surfaces like lime plaster, brick, or stone, creating a long-lasting finish. Very durable outdoors, especially on historic masonry. Requires a mineral substrate and often a special primer for best results.
- Casein paint - Uses milk protein (casein) as a natural binder, with lime or chalk fillers. Traditionally used on interiors for its soft finish and breathability. Suitable for lime plaster and older walls, but less durable in damp or exterior conditions.
A Word of Caution
Not all paints labelled as “breathable” truly are. Some petrochemical based emulsions or masonry paints use the term loosely, but their Sd values are often far too high to be considered vapour-open.
These coatings may allow minimal water vapour movement but still trap enough moisture to cause damp or peeling over time. Always check the technical data sheet for Sd values and stick to natural, mineral-based options where possible.
See Also: Breathable paint for lime render | Breathable paint for cellar walls | What is lime paint?
DIY Vs Professional Help
Lime plaster patch repair doesn’t have to be daunting. By using breathable fillers and paints, you can fix cracks, holes and small blown sections without compromising the character or health of your home.
Save the bigger jobs for professionals, but for everyday patching the right materials will give you durable, sympathetic results.
Need help and advice with your next project? Contact Us for bespoke advice on how best to use lime fillers in your next project, or for information on the best paints to use over lime plaster.
