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OK then I see no reason why these cant be parged without any prior treatment.
You could possibly do the parge with a broom and a sloppy mix? All it has to do is fill up all the holes, gaps and cracks. It is quicker and easier with a trowel though
try using lime and do it yourself, its easy. Do a test!
wet a patch of the wall and trowel on a bit of lime and see how it does. No reason why it shouldnt be ok. Then you can show it to the plasterer to give him courage.
The feeling is then the blockwork is okay to apply a lime based parge coat too without any additional preparation? No risk of drying out too quickly or falling off the wall?
The more I read the more I am thinking of doing this myself. However, I am concerned that I do this properly. The issue seems to be that the concrete blocks will take too much water out of the lime before it cures. I found this explanation on the Lime Technology website:
1. Masonry 1.1 Clay Bricks, Concrete Blocks Internal: • Apply Basecoat “wet-in-wet”, roughen up • After curing/drying time (mm per day) apply decorative finish External: • Apply Basecoat “wet-in-wet”, roughen up, • After curing/drying time (mm per Day) apply decorative finish
1.2 Aircrete Aircrete blockwork should always be dry prior to plastering. Measures should be taken to avoid its direct exposure to wet weather. Aircrete blockwork has a very high water absorbency. This can cause freshly applied plasters to shrink and crack through rapid dehydration. Unfortunately, it is common practice to remedy this by saturating the aircrete with plenty of water. This actually creates more problems than it supposedly cures.
• Unless the aircete has reached full saturation, the absorbency performance will only continue to increase and not reduce. To demonstrate this, compare what happens when a wet sponge and a dry sponge are placed upon a waters surface. The wet sponge readily absorbs more water and sinks quickly into the water, while the dry one floats upon the waters surface.
• Moisture introduced into the aircrete block remains trapped within the cellular structure and causes it to expand in volume. The moisture is eventually released, but over a long time period (up to 3 years). During this time the block gradually shrinks back to its original size. If the aircrete blockwork has been plastered while wet, shear forces will occur at the block and plaster interface, resulting in the plaster “shelling off”
However, the wet-in-wet "solution" states a minimum depth of 10mm which is too think for a parge coat. Am I being paranoid? Could I reduce the issue with an intial coating of something else (paint, limewash, etc, etc)
Umm what Tony said ;o) Looking at medium block and aircrete I would make the assumption it is former. Therefore, the absorbency would be less of an issue. How think is a parge coat supposed to be? I assumed it was pretty thin just enough to create a "skin" but some sites are stating 10mm.
If I go with the advice of Bot and use A bag of NHL (Non-hydraulic lime?) 3.5 lime mixed 2 sand to 1 lime what sort of consitency should I aim for.
For troweling on thin use it like finish plaster thick single cream for 10mm which is way too thick use like bricklaying mortar for brushing on thin single cream.
I've done one wall with a mixture of NHL 3.5, 1 part, and plastering sand, 2 parts. The mix consistency was so it just held itself on the hawk. Made quite a bit of mess but went on ok for a novice plasterer. I would guess a thickness of a couple of mm although difficult to tell as the block work is all over the place
I just read a few pages that say plastering sand may need washing? Have I messed up?
BTW I learnt that lime plaster is very painful if left on the skin and not washed off immediately!
Phew thanks Tony that is a relief. I've learnt the hard way about lime plaster. Was wearing latex gloves but in short sleeves so the plaster splashed on my arms. Now I have some sore forearms and minor burns.
I guess this is what they mean by your learn by experience!
To be honest plastering sand is a bit fine for use with lime - you're better off with sharp sand (sometimes sold as concreting sand) if you want a good, strong coat. If you're not going to put another coat of anything over the top it should be OK, however.
A parge coat, as I understand it, Is a very thin plaster coat to a wall with the intention of sealing it from an air perspective. It is around a couple of mm thick at most, bit like a skim coat, with the mix a consistency of single cream (thanks Tony ;o) Maybe a better description is when you put it on the hawk it just about stays on it.
In my situation I ave a modern house which is well insulated but the airtightness looks virtually non-existent. I have removed the plaster board filled gaps with expanding foam and then parge coated the internal block work with a lime plaster of NHL 3.5 and plastering sand. I agree with Tony about not wanting it any coarser as it simply wouldnt spread thinly enough (a couple of times I thought it was too coarse and it is possibly worth sieving as mine had a few stones :o(. I chose lime to allow the wall to still breathe and allow moisture to traverse and avoid gypsum or concrete.
Once again resurrecting an old thread, mainly because when I search on Google about lime parge coats this thread is top of the list.
I'm having a go at an integrated garage now using a 2:1 sand / lime parge. Last time around the consistency of the parge was such that it needed trowelling on and was akin to a finish plaster thickness. After re-reading Tony's comment I've tried a thinner mix and used a course brush (about the same as a yard brush) to apply what was a slurry.
I'm not sure if the parge coat this time around is thick enough so asking for comments:
Maybe this picture helps answer my own question, it is a closup of the wall at the edge. It looks like the parge has achieved a thin coating across the block which I assume is what I am looking for to achieve airtightness?
Thanks for the comments Tony. I think the brush may have been a bit too course and the mix a little too sloppy. I'll apply a second coat tomorrow using an external masonary brush instead of the brush in the pic.
Actually finding information on parge coats more so if using lime, does seem pretty hard.
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