First attempt at lime plastering - some questions

[mm289]

Hi all,
have spent the last week prepping and starting to plaster one of our rooms having removed all the cement render, repaired cracks, missing bricks and a load of stuff "hidden" under the cement render :shock:

Have been using a 1T bulk bag of scratch coat from Ty Mawr premixed with synthetic hair for the scratch coat. Questions are (pictures below):

- how "stiff" should the scratch coat be - as supplied in the bulk bag, even after 20-30 mins in a bell mixer it is a bit stiffer than I would use a lime mortar mix for bricklaying. After adding a little water (say 3 - 4 cups to a bell mixer) it is workable but after doing a couple of walls my wrists feel like they are broken as it is really hard to push on/flatten and not get to thick a base coat. I am reluctant to add to much water, but when you watch the MIke Wye DVD for example the scratch coat mix looks quite wet.
- having applied the scratch coat and let it dry out (gently over a week) I have some cracking appearing - which is OK, shrinkage cracks etc, but some of them when you press you can feel the plaster give, i.e. it is not bonded to the wall. The walls were well pre-wetted etc, but in some places it seems not to have taken. Is this normal - I guess not! In which case should I chop out these areas and re-plaster them?
- could the "Blooming" above have been caused by the plaster mix being to stiff and and not getting right into the brickwork etc?

I know it is really difficult to comment on things like consistency of mix when you cant see it - but helpful opinions are valued before I go ahead and apply the second coat.

Before:
View attachment 1
After:
View attachment 2
Cracks:


Many thanks,

Paul.

 

[OddJob]

To me it looks like one wall isn't clean enough,residue of paint?
There won't be any suction.
Lime plaster won't stick to brickwork without some suction and certainly won't stick to paint.
I would have raked out those joints to give a better key at least.
I would not have used synthetic hair nor would I watch a Mike Wye DVD unless there was nothing better on the comedy channel.
First base coat/scratch coat should be wet but not sloppy,it should be a workable mix and fluid enough not to break your wrist,a bit like a good bricklaying mortar.
Too stiff and it's just not workable.
You will hurt if it's too stiff.
First coat is kind of designed to crack but rarely does.
Allowing it to fully carbonize and crack if it's going to means your second coat won't crack.
If you lay a second coat over a still wet first coat cracks from the first will appear in the second and there is a danger the first coat will never dry properly.
If plaster has blown you will have to take it out and re-do that area.

Adding water?
Add enough to make it workable.
It's a bit of a fallacy that you add no water to premixed lime mortars and plasters based on a putty mix.
I should know,I spent quite a while making the stuff and slaking the lime.
Dependent on how old the mortar is it can be stiff but remember you can add not take away.
Personally I save lime water and throw it in the mix when needed,it fattens it up nicely.

I personally favour a very mature putty,too fresh and it doesn't coat the sand particles well which is more important than water.
Older the better for me.
A tip.
If you're using a mixer to knock up save the water and even the sludge and throw it back in next days mix.
Works wonders :wink:
Andy

 

[Penners]

OddJob - I hope Paul finds that advice useful. Personally I found it fascinating (even though I have no lime plastering planned!). You've obviously got a wealth of knowledge on the subject, and it's great that you're prepared to share it.

 

[OddJob]

OddJob - I hope Paul finds that advice useful. Personally I found it fascinating (even though I have no lime plastering planned!). You've obviously got a wealth of knowledge on the subject, and it's great that you're prepared to share it.

Penners,
I find lime fascinating and always have done,it wasn't until about 15 years ago I got to use it properly and have never looked back.
My "wealth of knowledge" as you put it comes from using it day in day out,not from reading books or watching DVDs.
I don't even know what a bag of cement looks like now!
As for sharing the knowledge my philosophy is if we don't we lose it,simple as that.

At 50 there isn't too many more bricklaying/plastering years left in me and trust me I ache like f**k at the end of every day.
So somebody has to crack on,only trouble is my current "apprentices" are 66 and 75 years old!
When they go so does what they know.
The younger generations don't want to learn.
Andy

 

[Penners]

my current "apprentices" are 66 and 75 years old!

Well, since I fall roughly into that same age bracket, maybe there's hope for me yet!



What? With my back?

 

[Schoolmarm]

I thought current advice was NOT to rake joints out because the coat of render then has too much of a variation of depth for setting. :?:

 

[OddJob]

I thought current advice was NOT to rake joints out because the coat of render then has too much of a variation of depth for setting. :?:

That may be the case in many situations but the wall on the left clearly has some kind of paint on it which unless it can be removed needs some kind of key for the plaster.
Depth of setting on a wall like that raked out shouldn't present any problems.
When you have severe variations like broken/spalled bricks/stones that can't be replaced the wall can simply be dubbed out where needed before applying the first coat.

 

[worms]

The younger generations don't want to learn.

While that may (or may not!) be true, I was under the impression that colleges taught bricklaying using lime mortar so that the sample walls could be taken apart and the materials could be re-used. If this is standard practice, apprentices will at least have some experience of working with lime mortar. Or is the place where I saw this unusual?

 

[OddJob]

They used a hydrated lime mortar,which is not a mortar that can be used in the "real world".
When they get out on site most either have the opinion or are taught that lime mortar is bad because it falls apart so easily,and of course this is the case with the stuff they use in college.
They are taught to use OPC and modern plasters and most won't even touch lime especially in plastering because of the drying times,they want to earn quick money,bonding coat one day and set the next.
Most bricklayers and plasterers will refuse to use lime or even have no knowledge of it.

 

[Schoolmarm]

They used a hydrated lime mortar,which is not a mortar that can be used in the "real world".
.

Why is it no good? People have used that who have posted on here.

 

[MdB]

They used a hydrated lime mortar,which is not a mortar that can be used in the "real world".
.

Why is it no good? People have used that who have posted on here.

Because they have a big vat of the stuff into which the used lime gets returned to at the end of the day to be reused. It is so old that any lime in it will be completely carbonated and is therefore useless at holding anything together for any length of time. (as told to me by my mate who taught plastering (gypsum) for a while)

 

[Schoolmarm]

Oh, so the lime mortar in college is rubbish but a hydrated lime mortar mix just used once on site would be ok.

 

[MdB]

Oh, so the lime mortar in college is rubbish but a hydrated lime mortar mix just used once on site would be ok.

It depends what for. If it is a job that requires strength then I would be very careful about using hydrate. I think I read somewhere that it has a strength equivalent to 0.2 to 0.5 so a fraction of NHL (I might be very wrong with these figures) meaning it isn't ideal for building a thin wall for example.

I would be interested in OddJobs thoughts between mortar made from hydrate and mortar made from putty. I've not noticed any difference between them in use though this may be down to my inexperience. Mixing seems to be the important thing as this makes a big difference to usability if you don't get it right.

 

[Schoolmarm]

I thought that putty and hydrate were as good as each other if the hydrate was new and not old stock.
And I also thought they were ok for building. We rebuilt our chimney breast with a lime putty mortar and it is not soft at all.

 

[MdB]

We rebuilt our chimney breast with a lime putty mortar and it is not soft at all.

(very much not an expert in this)
My understanding is that lime is very poor in tension. This means that thin items - such as thin walls are not great for lime as it won't take any sideways forces. Solid lumps (like chimneys) are not a problem as the forces on them are mainly in compression for which it is a lot better.

I would welcome corrections on this.

 

[FamilyWiggs]

OddJob - I hope Paul finds that advice useful. Personally I found it fascinating (even though I have no lime plastering planned!). You've obviously got a wealth of knowledge on the subject, and it's great that you're prepared to share it.

I second that. Andy - please post more.

 

[Schoolmarm]

Aha.

 

[FamilyWiggs]

Aha what?

 

[Flyfisher]

My understanding is that lime is very poor in tension.

Me too, but isn't the same true of cement mortars as well? (in general, that is - I appreciate cement will be stronger than lime). I can't think of any building applications where any mortar is required to perform under tension.

Incidentally, we once had a 70s-built house that had extremely soft mortar between the bricks. It was so sandy that it could be rubbed away in places with just a finger. I questioned the surveyor specifically about this and he said it quite normal and allowed for some settlement without cracking. I can't believe it was a lime-based mortar though, but it seemed very weak whatever it was. Or perhaps the builder was just saving on cement costs!

 

[Flyfisher]

Aha what?