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Testing the finish coat of clay plaster

The third coat of plaster is a bit of a different beast. For this layer, we are aiming for a thin skim of a few millimetres that doesn’t crack or crumble at all, and a surface texture and appearance that we’re happy with when it dries. Avoiding mould at this last stage is a good idea too as it can stain the plaster. It still needs to stick to the wall and have reasonable strength, so clay and fibres are involved, and much more sand. It is also a much wetter consistency, or ours was, to help spread it on thin. This is the realm of the plasterers’ trowel! Plastering with a trowel is surprisingly tiring and blisters/ calluses aren’t unheard of.

As this was nothing like the first two coats, we did lots and lots of testing in a much closer to scientific manner!

Our final mix was:

·         1 part earth

·         1.5 parts sand

·         ¼ part hair

·         200g borax

·         Water

There are lots of things to consider here – you might add pigments to the plaster itself to create the finish effect, you might add some sparkly bits like mica, you could get some posh coloured sand or crushed marble, all of which you’ll want to test out. While you’re at it, you can test the different options for finishing you are thinking about (paint, colour wash, glaze/ sealer).


You might also want to experiment with application methods/ finishing techniques. The main choice is between a polished, smooth almost marble like finish, or a sponged, textured finish. Different mixes will look and perform differently under different treatment here. We decided that the extra strength and wear-resistance of the polished finish appealed for the internal walls and we’d save our energy on the external walls and sponge that.

Polishing was VERY hard work, see the next step for notes on this!

A word on final coat clay plaster ingredients:

Clay/ earth – this time you want this as finely screened as possible, lumps and gravel will be awkward when trowelling on and can make for bumps in the plaster. We ended up spending extra time milling and sifting our earth this time round, it was worth the effort to get a smoother mix.

Sand – We got ‘plasterer’s sand’ from the builders merchant, which is apparently the finest they sold. Well, it wasn’t that fine really, not like sand-dune fine, but it did the job ok. You might want to investigate specialist suppliers – though if not local, delivery will make this extra expensive, but the effects created might be worth it.

Fibres – You may or may not need / want fibres, some people use very finely chopped straw which looks pretty as golden flecks, but we were done with clogging up the strimmer and weeding walls! Others use animal hair or synthethic fibres which you can buy or perhaps find locally. After some searching and asking around, it seemed the easiest and cheapest option for us was to get some hair from the barbers shops, which it seems is otherwise thrown away. This is better than from unisex or women’s salons because men’s hair tends to be shorter, and for this, short fibres are what we want. A little goes a long way. One big bag full form one barber was more than enough for all of our final coat.

Borax – This might not be necessary in your situation and previous experience with how the wall dried for the other coats will be a good indication. As we had a little mould on the other coats, we wanted to take action to prevent mould on the final coat, the tests showed this amount was effective, though we didn’t test without, and it may not have been necessary. Maybe you don’t need as much, but we didn’t have the patience to test out the mould threshold level for borax. There are some health concerns about using this, but no solid evidence and until recently it was touted as a natural cleaning product, so we weren’t too concerned.


Let the test patches dry thoroughly before assessing the results and look closely for crack levels. Interestingly, we found that the mix we tried with no hair at all was crack central, adding just a little hair really turned the mixture into something quite different. Perhaps this was a unique phenomenon with the kind of clay and sand we were using, perhaps not! Maybe if we’d added more sand this would have been overcome, but we were willing to go with the magical properties of hair. We tested out adding pigments to both the plaster and the glaze, but in the end decided we liked the natural colour the plaster dried to the best.

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