Waaaa! Lime finish coat cracks

[Anonymous]

Darn, darn, darn! I covered about 50 square feet of wall with finish plaster yesterday which began to crack almost right away and according to my friend doing the follow up water spraying, cracked off part of the wall after I left! Aaagghhh!

What the heck am I doing wrong?
I thought the cracks before were a result of having no one there to spray the walls after application for a slow cure. Apparently not.

I have a fine lime finish plaster-dry mix w/ sand already in.
I prepped the walls ahead by having them sprayed.
Covered my plaster washers with mesh tape embedded in putty.
I primed walls over all with a mix of lime putty and water.

Did my first ever Large Batch of a 50# bag with 1 gal, 12 oz water as recommended. Used a hand mixer that attaches to a drill. (Not fun. I kind of liked kicking a small batch bag around better.) Water in first, then plaster mix. 20 min of mixing (plus more water) to get a workable product.

A. First problem-mix balled up like lots of walnuts--very dry. Added some more water. And more... *Finally* got a product that would stick to trowel and not fall off AND stick to wall without falling off if I flung it.

Question one: what is the maximum amount of water that you would DARE add to a 50 pound bag of lime mix to get workability? Maybe that 140 oz water was somehow supposed to incorporate if I mixed it Forever and would eventually get all nice and spreadable. Maybe I need a more powerful kind of mixer. Maybe I should stick to small batches.

B. What to use to put finish coat on wall--I used a mixing trowel held an an angle to get the stuff on the wall and spread it out, intending to go over the result with a larger finish trowel (knock down and smooth)

Problem: Looks like a Van Gogh painting in white! Very RUSTIC and wavy. I'm very slow putting on the plaster and working my way up or down the wall in sections. What are the signs in plaster that it is ready to get "knocked down"? Or is this something one doesn't do with a finish coat? You're supposed to put it on smooth?

Anyway, I went back over the waves and filled in the catfaces, and low spots. I thought I was getting good adherence with the plaster wall behind (which is a combo of places where I had already done 2 coats of repair plaster plus the old surface of the original wall left intact, but paint mostly removed).

Major cracks constantly appeared in the previous work as I moved to other areas of the wall to use up my plaster.
Should I have used a wood float to put on the coat?

Last, I keep getting black burnish marks on the high areas of plaster when I use the finish trowel. Why?

OK, can some experts weigh in on all the things I'm doing wrong in my quest to learn througn trial and error?? What should I do with this mess when I return on the weekend? Friend says "It ain't pretty." Sigh.

***************
To the other poster looking for hair to add to lime in the US--try Virginia Limeworks. They have goat hair you can purchase. I used white.

 

[Anonymous]

"plaster washers"? "mesh tape"? ... What exactly were you applying your finish coat on top of? I suspect the answer to this question might be the key to your problem

 

[Anonymous]

how wet was the wall you plastered on to ? Could it be that its sucking the moisture out of your top coat too quickly ?

What exactly is this 'lime mix' ?

I dont like the sound of the mix, starting dry adding a little water and mixing with a drill. I would expect (in my admittedly limited experience) to have a very wet plaster - yours sounds dry to me. 50sq m is quite a big area to go for as a starter as well. Very difficult if you are not used to plastering to get a good finish.

I would try experimenting with a couple of square meters and see how you get on. My lime plaster takes ages to properly dry out (but yes I do wet the surface to keep it from drying too quickly).

 

[Anonymous]

Plaster washers: small flat perforated discs through which one places a screw to adhere loose plaster to the underlying support structure.
Mesh tape: Used to cover the metal of the washers.
Wall surface: Lime plaster over 100 years old, paint removed, but with a water repellent gloss of some kind under the paint, recently applied coarse lime plaster-"brown coat".
I did make a 1:8 mix of lime putty and water over the wall before plastering as a bonding agent instead of the pink chemical stuff.

 

[Anonymous]

I have a pump sprayer. Wall had been sprayed and was sprayed often in the process.

Mix is a dry premixed NHL plaster with fine sand made specifically for historic preservation. It comes in bags or buckets. ( Putty is very hard to come by--I'm lucky to have some coarse grade, but mine isn't suitable for finish plaster, I'm told.) Not even sure how I'd go about mixing sand and putty without a roller mixer which we don't seem to have here for home DIYers in the US. Apparently one needs to BOTH mix and BEAT lime plasters/mortars which is what the pan roller mixer does and cement style mixers don't do.

If you are using lime, what consistency is your finish coat? You say wetter, but you don't get shrinkage cracks?

Maybe I should stick to small batches. :-/

 

[Anonymous]

I bought my plaster premixed, in tubs. As I understand it its just putty and fine sand. It came very wet (as in layer of water over the top wet). I took out enough for a small area at a time, worked it with a trowel until it had a consistency like very thick cream (which is what I would do with modern plasters) and applied it. The only cracks I got are very fine hairline surface cracks that polished out and a couple of larger (but still small) cracks where I built up a deep layer of plaster to turn a brick step into an arch (so expected cracks because I should have done it in layers rather than 1 go).

Is it just your top coat coming off or is it every thing down to the gloss paint stuff?

 

[Anonymous]

Call me old fashioned but I've never mixed lime plaster/mortar with any tool other than a spade. Saves going to the gym. One advantage of dry hydrate powder over putty is that one can mix a barrowful in ten numites instead of twenty.

 

[Anonymous]

Geez. You mean like yoghurt consistency that sticks to a spoon? I assume it just glides on the wall then, flatly.

See, I'm dealing with someting between cookie dough and a stiff peanut butter that really requires a lot of muscle strength to push around on the wall. There's very little that's smooth and glidey about it. I keep reading that too much water can cause cracking, so I want to figure out if I *am* adding too much water to get to a spreadable stage. I wonder if continued mixing of some sort would bring it to that consistency that you have described while adding no more water than that gal+.

I have mixed outdoor mortar with a hoe and I understand that the chopping motion is good. I do that on a very small scale with a gauging trowel when mixing small amounts of my base and brown coats. Then into the contractor bag to beat up! But, I do always get that stiff consistency-NOT like a cream.

I guess I'm trying to figure out if the finish coat OUGHT to be moister and easier to work than the other 2 were. I know the sand is different sizes, so it is definitely finer to work with. It's the consistency that's throwing me--I have nothing to look at or feel to know what's right.

 

[Anonymous]

Please tell me if I am wrong, but when I did a plastering course (your modern stuff) we were taught to put the plaster into the water not the other way round? Is it different with lime plaster?

 

[Anonymous]

Your plaster sounds too dry and the wall must be thoroughly wetted and keyed first. If the plaster is wetter you might find it easier to apply it with a steel float rather than a trowel.

 

[Anonymous]

'water repellant' + 'nhl lime'..... sounds like the root of the problem.

 

[Anonymous]

It's starting to sound like some odd patented mix which isn't lime plaster as we know it...

 

[Anonymous]

What are you in the UK using? NHL-natural hydraulic lime. Eco-friendly, natural material, "lime produced by burning argillaceous or siliceous limestones with reduction to powder by slaking with or without grinding." No cement added.

The company offers a traditional lime putty made of high calcium limestone or oyster shells. Is this what you are preferring to use? Why would the NHL (once mixed with water and worked) and calc lime be of different consistency?

 

[Anonymous]

We use as far as possible non-hydraulic for plaster - and most other purposes unless it's really, really exposed or needs a chemical set or under water!!

Hydraulic isn't very breathable - apart from the eco-friendliness you may as well use cement...

 

[Anonymous]

Some very basic reading...


http://www.spab.org.uk/publications_Q&A_lime.html

 

[Anonymous]

Interesting. This company does historic restoration work in Williamsburg, VA and other spots in the US. I did consult with them to find out what I should use and they recommended the dry mix rather than the putty.

I do know that it's weak natural hydraulic line that I am using in the bags, and that there is free lime that will self heal hairline cracks by action of atmospheric CO2 with air moisture. Their website does say specifically that NHLs are vapor permeable allowing buildings to bring in and release vapor and moisture.
This is also what the St. Astier site says reermeability of NHL (vapor exchange and condensation dispersion) which is what I have assumed "breathability" to be. Their (Astier) NHL IS said to be high calcium, so I guess the consistency issue I'm facing must really have to do with starting with a putty rather than a dry mix.

Thanks for your responses, Evelyn.

 

[Anonymous]

Weeell - they would say that wouldn't they! I think that using hydraulic for plastering may not be helping however. I think it may have its uses for pointing but....

Some further reading... use the links to other articles part....


http://www.buildingconservation.com/articles/plaster/plaster.htm

 

[Anonymous]

You know, I did a Google search on non-hyraulic limes and every source or use link came up UK or referenced it.

Observing that people historicallyused what was closest to them in order to avoid excessive transport costs, it makes me wonder if non-hydraulic limes were easy to come by here in the US. *Maybe* close to the ocean ( thinking oyster shell deposited limes)--but what about inland by a few hundred miles? If the resource was/is easy to come by, why wouldn't we have companies in the US who offer such to historic specialists instead of needing Mike Wye as an importer?

Perhaps the lime plastering experience for the common man may have been slightly different here in the US because our natural lime mineral resources differ from those available in Europe (?)

 

[Anonymous]

Just an idea but I used to live in Colorado and you dont get much further from the sea up at 5280 ft in Denver, I used to visit the old victorian mining towns of Golden, Aspen, Black Hawk or Nederland and many of the buildings are now restored. You could try and contact the heritage people at one of these places and they may be able to help. Try the Weeler Opera House

I have seen old timber cabins with a form of render at the joints which must be from before the days of cement in these areas. I have some books at home and later I will see if I can see some numbers for you



National Trust USA

 

[Anonymous]

There are also all the technical information publications from the Office of Cultural Resources, National Park Service, USA - maybe there is help avialble from there?