metal 'brings lime to the surface'

[Mark]

I keep reading this comment that metal tools bring lime to the surface of mortar.

How does that work then ? I dont understand how just using a metal tool will miraculously bring lime to the suface of the mortar. Is it that a smoother finish on a tool doesnt strip off lime from whatever aggregate is involved?

 

[Gervase]

It's a bit like when you have a jar of sweets or pebbles and shake it and agitate it - you'll see they grade themselves, with the heaviest at the top.
As you agitate the plaster or render, you end up bringing the lime to the surface. That's why, when working plaster, it's best to use a wooden float until you need to get the final polish. Too much working with a steel float can strip so much lime from the back of the coat that it loses its adhesion!

 

[biffvernon]

The shaken sweet jar analogy doesn't really explain why steel and wooden floats give different results.

 

[Penners]

Here's a theory from a total lime and plastering ignoramus, so it's probably complete rubbish:

Wooden floats generally tend to be used for smoothing off. Steel floats tend to be used for a final polish. To me this implies less pressure on the plaster from a wooden float than with a steel float. So could it be that the higher pressure from a steel float squeezes the fats out of the mix and up to the surface?

Please be gentle with me, you experts...

 

[Gervase]

With steel floats there's certainly a suction effect that you don't get with wooden floats (unless the plaster is very wet) and that almost certainly has a bearing on matters.

 

[Mark]

Hmm, So thats a resounding 'not sure' then ?

Suction is an interesting thought - but presumably would vary with the technique of the user and the angle the float sits against the wall at.

Steel tends to be more flexible and smooth, which could have an effect.

What about plastic floats? tend to hvae a textured surface - how does that compare to a wooden float?

Assuming the effect does exist, has anyone experience of the range of different tools ? Does the result vary with the size of the aggregate - silver sand vs sharp sand ?

thanks
M

 

[BMcD]

A metal trowel will bring the fat to the surface because it closes up the gaps between the particles forcing the finer/wetter binder to the surface.
A wooden/plastic float will compact the material but leave an open grain allowing the binder to sit below the surface.

The larger the gaps between the particles and the wetter the mix the more the effect can be evident, for example if you over trowel concrete it will form a laitance scum on the surface where the water has brought with it the binder especially if it is too wet when it will form puddles in the slump depressions

 

[Moreteyne]

My experience would indicate that any trowel is good, just ensure your plaster is not overworked.

 

[Mark]

Sounds like someone needs to try some experimentation here.

(Not looking at anyone in particular, not even those with a history of experimentation, no really, not even looking at Biff, not even a little bit...)

I think I like a combination of Penners story woth BMcD's.
squeezing rather than suction and smooth surface rather than textured surface.

Gervase, have you experienced the mortar falling off after excess working ?

 

[Gervase]

I've experienced it with plaster when I was doing a series of experiments on plasterboard. I realised that the plaster had to be compacted really well to give a coherent coat, but found that a steel float tended to bring the lime to the surface and affect the bond between the plaster and the board underneath. The stuff that came away was coarse at the back and very limey at the front.
The upshot of that was to use two coats of the dreaded PVA on the board, and to work the plaster hard with a wooden float - saving the steel for the final polish. Then it did stick.

 

[Penners]

But presumably, Gervase, you wouldn't advocate using PVA as part of the process when the wall needs to breathe?

Mark - please don't attribute any authoritative opinion to me, in this matter. I have absolutely no expertise or knowledge of lime plastering. My suggestion was simply inexpert speculation.

 

[Mark]

Penners,

perish the thought!

I'm looking for reasons that 'make sense'. There seems to be a lot said and commonly understood about aspects of building that pass for absolute truth without any justification. I like to at least think that I understand why I am doing things in a particular way. As a starter the common statement that steel floats bring lime to the surface is repeated all the time - without any explaination. Im looking for a physical or chemical justification. The idea that lime is magically attracted to steel is obvious rot (dry/rising take your pick So it has to be either myth or something about the nature of the working or tools.

I have only done little bits of lime plaster (and I used metal tools because thats what I have). I have used premixed in tubs, self mixed from putty and self mix from dry bagged. It all seems to work, which is against the perceived wisdom, but I saw reports from Biff that it was fine and indeed my render hasnt fallen off yet despite being 1 bagged lime : 4 or 5 sand : 1 soil.
I have noticed differences in workability with different mixes, and I have had trouble getting some stuff to stick, but then I have been going onto spalled bricks, chimney bricks impregnated with soot, a bit of skim over gypsum, etc. It will take ages for me to get the experience as a diy'er to really understand the best techniques, but Id like to understand rather than just do as im told (prob why I never made a good employee and have to work for myself)

In other areas of exertise you often find that one way of doing something becomes the 'only' way because thats how its done. Then some other technique turns out to work perfectly well despite the tooth sucking and head shaking of the experts.

You only have to look at the whole damp issue and the arguments about breathable buildings....

Anyway it is useful to hear that problems can happen and tyat peole have seen problems with steel tools - although you do have to wonder about the plasterboard factor - untill I see a textured steel float I wont know. And I promise not to ascribe any knowledge to you

Mark

 

[Penners]

There seems to be a lot said and commonly understood about aspects of building that pass for absolute truth without any justification

You're dead right. I suppose that any craft/skill that's as old as building is bound to develop "old wives' tales" that are handed down through the generations. It's certainly a good thing to question them every now and then, and even test them empirically.

against the perceived wisdom, but I saw reports from Biff that it was fine

Biff invariably seems to have a way of cutting through the "religious beliefs", and finding his own common-sense approach to traditional methods.

 

[Gervase]

But presumably, Gervase, you wouldn't advocate using PVA as part of the process when the wall needs to breathe?

Absolutely not! I would only ever use it where lime plaster was being used cosmetically over plasterboard - a PVA tack coat in normal circumstances would defeat the purpose of the plaster.

 

[Mark]

Biff invariably seems to have a way of cutting through the "religious beliefs", and finding his own common-sense approach to traditional methods.

Exactly. Traditional beliefs started for good reasons and people would have understood why, then you get into rote learning and suddenly everybody is doing it but nobody knows why.



Mark

Founder member : The Biff Vernon Appreciation Society

 

[biffvernon]

I'm sure the feeling's mutual.

Biff

Founder member : The Mark Appreciation Society


PS I don't know why but I do know that my sleel float tends to leave lime plaster smooth and my wooden float tends to leave it rough. I also have a textured plastic float but it just leaves a mess so I use it as a small light-weight hawk. Both steel and wood feel equally smooth to the touch so I guess it must be some other surface property. We probably need to get down to the level of electrical effects and surface tension.

 

[Mark]

Nothing ickier than a mutual love-in.

Moving swiftly on.... I had that problem with the plastic float, and it now serves exactly the same use.

I did wonder if the combination of smoothness of surface and hardness of material makes the difference. I was thinking in terms of the sand dragging on the float, steel slides, wood should scratch/bend/something? plastic catches.

Open to further suggestions though...

 

[Penners]

As regards the plaster sticking to a wood float, I reckon the absorbency of wood must be blame, mustn't it?

I just stated the flippin' obvious, didn't I?

 

[monkey writer]

There seems to be a lot said and commonly understood about aspects of building that pass for absolute truth without any justification

Any time you speak to a builder or plumber you come up against this. For instance we're getting our combi boiler replaced. Every single plumber has said we like to install Worcester Bosch. Why? Oh because they're the best, most reliable. Well I've done my research and I know that Which gave them a bad report and that they have aluminium heat exchangers, while (Kent based) Alpha have stainless steel exchangers for much the same price and with a longer guarantee. Oh, I don't know, they say, Worcester have always been the best... But they have no evidence or reasons to back it up.

So we found a plumber willing to give Alphas a go. Fingers crossed.

 

[Penners]

That's called "the power of advertising". If you tell people often enough that your product is the best, it can eventually become a mantra.