Lime plaster

[El Greyo]

Hello all

As I've been researching lime plaster, I've been pondering why it is standard practice (as far as I'm aware) to apply two layers, a scratch and a float before the final skim layer. There must be a good reason for it but I can't work it out. Lime plaster includes horsehair (or similar) to strengthen it but I would have thought that, despite the scratches on the scratch layer, the joint between the two layers introduces a weakness. Is it something to do with mitigating shrinkage?

Can anyone enlighten me?

 

[robgil]

Ive been using a coat and a half, the scratch coat is covered with another coat when it is still green. Works for me.

 

[tobydog]

We recently had our inglenook renovated. The half of it that resided in the old kitchen was completely covered in cement render. We had to hack the cement off at the expense of several brick surfaces, however, it was worth it because the lower bricks were saturated. After 3 summer months of allowing the bricks to dry significantly, the plasterer used a one coat lime plaster that set in a day or two. It is almost white and the finish is quite hard. In hindsight this was necessary because several parts of the brickwork were unstable where the locked in moisture had done its damage.

We didn't have a say in the plaster because, to be honest, I didn't know one type from the other at that stage. I was surprised that it only took one coat. I vaguely remember it being made by Castle and originating in a famous French limeworks. The finish, although quite rough, is very good and there are no adverse reactions on the bare inside face of the inglenook.

I have been concerned that maybe we used the wrong stuff and that the lime is probably harder than the surfaces it is clinging to which technically is wrong. Only time will tell but so far (12 months) so good.

 

[michael]

Not being a plasterer, I'm not really sure but is it perhaps to stop slumping?

 

[GRB]

I think that three coat work has more to with the very best work. This is the best method to get very straight walls and ceilings, which plasterers of old used to pride themselves on. Two coat or coat and a half work was considered fine for rustic interiors and agricultural buildings, but these still create a perfectly appropriate finish if done well.

Regards the strength, the three coat work that I have had to remove in the past (only where it has failed for other reasons, such as decades of water pouring down it!) has certainly never peeled off in the layers that it was put on.

A good book to explain plastering, written in the 19th century, is Millar’s Plastering Plain and Decorative -

http://www.archive.org/stream/plasteringplaind00mill#page/n0/mode/2up

 

[El Greyo]

Thanks very much for the replies. I had assumed I would be using three layers as that is the best thing to do. But if a sound job can be done with one and a half coats, then it would be much quicker. Perfectly straight walls are not only not necessary in our cottage but would be out of keeping.

Any tips for lime plastering gratefully received.

 

[MatthewC]

Any tips for lime plastering gratefully received.

Since I'm also near Oxford, if you would like the details of my young and now rather more experienced plasterer, PM me.

Matthew