First, read our introduction to clay plasters. Now here are some common questions you might need answering before considering clay plasters.
Where can they be used?
On any internal walls and ceilings. They work particularly well with other natural materials like cob, rammed earth, straw, hemp etc.
Do they contain any synthetic ingredients?
No – only natural, non-toxic, biodegradable ones.
Do they need to be painted?
No – the top coat is supplied with a range of pigments included within the plaster.
Can clay plasters be used externally?
Only if they are protected by a roof – e.g. a porch.
Are there any internal areas where clay plaster shouldn’t be used?
Water splashback areas – e.g. around baths or sinks. Use tiles here.
2. Preparation
Will the surface be smooth or textured?
If you use a plastic trowel only, the surface will be smooth; or you can use a damp sponge for a textured finish.
Will the surface be straight or curved / undulating?
Either – it’s up to you.
How much will 25kg cover?
Top coat: approximately 7-8 square metres at a thickness of 2-3mm.
Base coat: approximately 1 square metre at a thickness of 13-15mm.
How do I prepare walls for the plaster?
Make sure the underlying surface is slightly rough (score the base coat slightly, for example – not too deeply though, or it will show through). Moisten the underlying surface with a fine mist spray before applying – the more absorbent the suface, the more water should be applied – e.g. constant moistening when applying over cob or lime. Also, the higher the temperature, the more moisture should be applied.
How much water do I mix with the plaster?
About 8 litres per 25kg, but slightly more or less depending on the absorbency of the wall – more if the wall is very absorbent.
How do I mix the plaster?
The plaster is non-toxic, but it’s best to avoid breathing in any kind of dust – so wear a mask. Mix in a bucket with a plasterer’s whisk.
1. Pour the dry powder contents into a bucket.
2. Blend the dry powder with a plasterer’s whisk, as the ingredients may have settled during transit.
3. Add approx. 4 litres of clean water (out of the total 8 litres required) to a separate clean bucket.
4. Add the dry powder to the water in small increments, mixing with a plasterer’s whisk as you add it.
5. Add a further 3.5 litres of water, one litre at a time, as you add the dry powder, until the desired laying on consistency is achieved. The plaster should resemble a mousse-like consistency; wet enough to flow smoothly off the trowel and onto the wall, but firm enough to hold it’s form on the hawk.
After initial mixing, let the mix sit for five minutes to ‘tighten up’.
Finally, add the remaining 0.5 litres of water, to bring it back to the original consistency. The mix will ‘tighten up’ in the bucket over time. It may need more water to maintain its consistency. Throughout the plastering session, monitor and add water according to personal preference.
There are natural additives present in the plaster. These have a shelf life of 1-2 days once mixed with water. Only mix what you will plaster that day.
Retain surplus dry powder for repairs. Store it in a sealed container, somewhere cool, dry and dark. Alternatively, you can safely compost.
Tools should be cleaned thoroughly with water after use.
Thanks to Clayworks for information.
