Learning 3 coat lime plaster

[Anonymous]

Hope you don't mind my posting--I'm in the US. It seems that whenever I'm searching for an answer to my rehab, UK sites are the main resources that I find. Thank you!

I have a farmhouse from either the late 1700's or early 1800's, built with logs and *****ing originally. Later it was covered over with a brick veneer. The interior lime plaster walls over split lath were also added later, some time in the 1800's. It has both vertical and hortizontal lath.

Much settling has occurred and there are numerous wall cracks and bulging from sagging lath, especially in the ceilings. I've already enlarged and filled some wall cracks and pulled loose plaster off the wall down to lath, filling with 2 coats of special haired plaster. I plan to coat the entire wall on each side of the rooms with a skim fine lime plaster coat.

In the US there is almost no one who works with lime plaster any more. Those that do know, guard the practice like a military secret. There are only a few places that provide real lime (not gypsum) plaster with the appropriate sand gradations. I don't have anyone to show me whether I'm mixing correctly (from a dry mix) for my circumstances. I so wish I had a video that would cover my questions.

For those of you who have worked a 3 coat plaster job, how did you judge if you had achieved appropriate plasticity in each layer as you were mixing scratch, brown and finish?

If mixing by hand (I'm working small batches of around 4 QT dry mix to 1 QT water), how long should one mix with the gauging trowel after the wet and dry are totally blended. I use a chopping motion for a while, but don't know when it's enough. How much moisture should a mix have beyond what the package states? It seems as though my last batch was like working with ,mostly sharp sand--it did not want to hold together and dropped promptly off my trowel though it would push into place and hold if worked in very small trowel loads with me catching up what fell off.

Sorry this is so long, but I do so need advice!

 

[Anonymous]

One thing about lime plaster is that it is extremely easy to over-wet it - and if you add too much water you'll have to add considerably more dry matter to soak it up. If the mix goes on the wall or ceiling too wet it will crack, and crack badly.*
Lime plasters should always be much 'stiffer' than their gypsum equivalents - it should be wet enough to take a smooth surface from a trowel as you smear it on the hawk, but not so wet that it 'slumps' under its own weight when you first slap it on the hawk.
It also needs a lot of mixing - far more than you think. Being slightly thixotropic, lime plaster will become 'slicker' the more it's mixed, and it's all to easy to add more water early on in the mixing process only to find that the more you beat it up, the runnier it gets.
Mixing on a spot board with the trowel is, I'm afraid, by far the hardest way to mix it! If you can beg, steal or borrow a powered mixer, life will become much less stressful very quickly. Failing that, put the mix into a very heavy-duty polythene bag or rubble sack and kick the#####out of it - the more mixing and beating up, the better.

*the first coat will always crack - dont worry about it. If the cracks are huge and you can see the substrate, close them up with the trowel as the plaster stiffens. Most importantly, key the surface with a comb or pointed sticks before it sets too hard - the better key you have, the better the second coat will stick and the less liable it will be to crack.
The second coat is more crack-critical - aim to close up all cracks with a wooden float, spraying the surface with a mist of water as the plaster hardens. Score the surface with a comb or devil float before it fully hardens to give your finish coat a good key.

 

[Anonymous]

Mike wye has a video/DVD which I am sure can be posted to the USA...


http://www.mikewye.co.uk

 

[Anonymous]

.......... best not to use a modern scratch comb; the scratch key tends to crumble in when pressured.... hence 'historic' plaster invariably has key marks akin to those made by a rounded stick at larger centres. also important to wet the substrate prior to first coat and to ensure the same thereafter... and ensure that the stuff does not dry too rapidly. also, keep cats away from the sand pile to avoid the obvious : - )

 

[Anonymous]

Same situation here, I'm in the US. Would you mind telling me where you get lime mortar or lime putty in the US?

Thanks,
Bernhard

bhoublon@yahoo.com

 

[Anonymous]

Sure.
I've been buying it from Virginia Limeworks. I don't know where you are, but they have recently added some distributors to their website, so was was able to save on the shipping by picking it up in Washington DC instead.

THey sell a product called MixNGo (dry) which I have been using along with goat hair which you can buy from them.

Thanks to everyone who responded!

 

[Anonymous]

Wow,
I think I've been undermixing then and adding too much water.
My lath is very old and it seems as though I can spray it forever and not see much wetting. It sucks the moisture right out of the base coat. Then I wet down the base (scratch) coat but am getting lousy adherence from the brown coat which doesan't spread worth a darn without disengaging from the wall. Thinking I needed more moisture, it spread smoothly, but would not stick to the base coat. Air got behind the "beautiful" smooth job, it sagged and a crack appeared. Then a large chunk looking like I'd rolled it out with a rolling pin just flopped off the wall. I sure do have a learning curve. I did purchase the only mixer I could find that fits on the end of a drill, but my drill was too small.

 

[Anonymous]

Aye, sounds familiar!
I think it's something that anyone who's ever started lime plastering has experienced. Unfortunately it's one of those skills that's very hard to impart at a distance - like a lot of craft trades, the only way to learn is actually to do it.
The first attempts I made were absolutely dreadful - in fact I've now chipped them off and redone them because I was so embarrassed.
Don't be disheartened - it's Zen-like thing; the less you worry about it, the better it becomes. It does take some physical strength, however - you have to get used to really pushing the stuff home to exclude the air. On a non-lath surface (or for the second coat) the best way of doing that is to use the trowel and literally hurl the stuff at the wall. It'll make a satisfying splat and will stick like snot to an oven door, needing just to be spread out to join up with your other splats.
Have a look at the pdf below which gives a brief intro to the topic...


THREE COAT LIME PLASTER ON RIVEN LATH

 

[Anonymous]

...also, if you're still having problems getting plaster to stick to the scratch coat, try wetting it with limewash rather than water - that can make quite a difference.

 

[Anonymous]

I taught myself lime plastering from an excellent book entitled "Building with Lime" which I bought from the SPAB bookshop (SPAB = Society for the Protection of Ancient Buildings, a UK based charity cofounded by the Arts & Crafts pioneer William Morris), plus Wike Wye's video (UK videos dont always work in the US so check before buying).

I started by using ready mixed lime plaster from Mike Wye, but I now mix my own for small jobs. I found that controlling the amount of water in the plaster is not a major problem if you make the plaster from lime putty. If the sand is moist you may not need any additional water; if dry some additional water is needed, but not much.

Wetting the wall or lath is essential and the book recommends that it is started some time in advance of the plastering (a few hours) to allow the water to soak in. Lath on the whole needs less damping than brick. The worst problem I had with suction was modern breize block which had been used by my builder to reconstruct part of my chimney breast. To get over this I harled it first (harling involves litterally throwing on highly diluted lime plaster with a special trowel). This is not suitable for lath however.

For lime plaster to set properly it should be kept damp for several days. The US has much drier air than the UK, so I suspect that you would need to pay particular attention to this.

Good luck

Nigel, London

 

[Anonymous]

You all are so incredibly helpful!
Regarding limewash--is this a product that is purchased or that one makes oneself? Unless I special order, the only lime I can find is either garden lime or the stuff that is not heat treated at high temperatures. What do you do to get a finish coat to adhere to the old wall? It has some kind of finish (sealer?) that makes water bead up and run off. I'm still trying to remove areas of paint that refuse to be budged. Here in the US they advocate using a chemical brush-on bonder. I don't know that lime will adhere to the plaster of paris patches previous owners made.

I can purchase lime putty from the same supplier, but I'm not sure what to do about the sand. They indicate that each layer has percentages of different sizes of screened sand. ( It does look as though my historic plaster may have used something closer to silt than sandy though.)

I have 9 more rooms to work on, so I should eventually get much practice!!

 

[Anonymous]

Base coat plaster is about 3 parts sand to one of putty. For this plaster a fairly coarse sand is generally used (often with quite a range of aggregate size), plus hair. Finishing plaster is more like 2 to 1 or even 3 to 2 sandutty. For this a fine sand is used. In the UK this fine sand is known as "silver sand".

The great thing about lime putty is that it doesnt go off (so long as it is kept wet). In fact it is supposed to improve with age. The putty I am using now is at least four years old.

 

[Anonymous]

Make your own limewash with lime putty - it's much easier and cheaper than buying the stuff ready-made, and you have more control.
For example, for wetting plaster to improve the key when adding a new coat you need a fairly dilute wash - say, one part lime putty to eight parts water.
But for limewashing a finished wall you could use a ratio of one part putty to three parts water or even 1:1, depending on how absorbent the substrate was.
Don't use any sort of bonder - it will probably be PVA based and will not be breathable. If you have problems with your finish coat sticking, go over the second coat with a wire brush, make sure it's well-wetted and then give it a coat of dilute limewash just before plastering.

 

[Anonymous]

Hi All

I just took delivery of a ton of loose "double washed sand" from a "specialist" local lime supplier. It is bright orange and leaves an orange stain on your hand. The builders are not impressed. If you mix some with water it exudes a muddy orange colour like dark tea, which makes me believe it is not washed at all.
I have to wash it about ten times before the water runs clear.

I've had two bad experiences of using lime with orange sand. In the first it left a nasty brown stain leaking through layers of limewash, irrespective of number of coatings. I had to dig it out and start again.

In the second, an inappropriate use of the orange sand in lime mortar repointing (by a "brick expert") means that pretty much a whole wall is permanently brown and discoloured.

Does anyone know what causes the orange colour in sand (is it a mixture of clay fines and somethng else?). Also I just tried two other sources of "washed" plastering sand and both give off a muddy orange when washed.

Is there a difference between "normal" washed plastering sand used by contemporary non-lime builders and the washed sand you need for limework?

Cheers
Nick

 

[Anonymous]

The yellow/brown/orange/red colours in sand are probbaly iron oxides. Should be quite harmless and are sometimes deliberately added to lime mortar to stop it being so bright white. But it sounds like you didn't get 'washed' sand. A certain amount of clay doesn't harm lime mortar either.

 

[Anonymous]

I had a similar experience with a bag of "plastering sand" from my local builder's merchant. It was less of problem for me because I only used it for base coat; my top coat silver sand was quite pale and clean.

 

[Anonymous]

Has anyone seen this dvd? Evelyn? If so, is it worth it? No substituion for a course, but worth while watching before going on one to get to grips with the ideas?

Also, as they are probably PAL dvds, you'd need to watch it on a computer in the USA. To watch it on a dvd player it would need to be standards converted to NTSC, which is something I do for a living - only with Mike Pye's permission of course <IMG SRC="http://www.periodproperty.co.uk/discussing/smileys/smile.gif" BORDER=0 ALT=""> Unless you are one of the few in the states who have a dvd player which can play a pseudo pal signal. More common here than over there.