Lime Plastering question

[Damon]

Hi all,

I'm in the process of replastering our cottage and luckily most of walls are still in good enough condition to only warrant a skim. They are all lime (phew) and have previously been covered with about 10 layers of wallpaper. I've removed the wallpaper and have got back to the lime and am now ready to skim. I know with gypsum plaster it's important to use a layer of unbond or PVA but I'm loathed to do that on wall that i want to keep breathable. On the walls I've had to replaster completely I've used a haired sharp sand and lime mix for the first coat, sharp sand and lime for the second coat and will be using a silver sand lime rich mix for the top skim.

What should I do to get the skim to stay put? Is there a breathable alternative to unibond? Shall I just score the walls to provide a mechanical key? I'm out of ideas as i do not want to compromise the breathability of the walls at all. The cottage is finally drying out after previous neglect and it feels great to be the one to save it!

Thanks very much,

Damon

 

[plasticpigeon]

I bought some ready mixed bagged lime skim that just needed re mixing to make it a smooth paste rather than a lump sitting in water, and it was quite sticky when mixed for a bit. It will be put in very thinly so I think the biggest problem is getting teh wall below wet enough. Don't use pva or unibond as that defeats the object of a lime plastered wall.

 

[88v8]

As PP says, the understratum must be wet.
Saw a lime plasterer working last summer, he had a pressure sprayer - one of those pump-up things for the garden - and was constantly damping both the wall and the new plaster as he wentalong.

In terms of what to put on, readymix is probably your friend, considering they have spent years developing the mix while you have to guess it. The small extra cost vs the hassle saved.............
https://www.lime.org.uk/products/lime/plasters-renders.html

Another thread on the topic
http://www.periodproperty.co.uk/forum/viewtopic.php?f=1&t=15178

Ivor

 

[Lime]

How bad are the walls?
Plain polyfilla will do the job very well if the walls are flat but holed and pitted.
PVA will not last long in comparison to the age of the house, perhaps 50 years if you are lucky, less if it is damp.
Don't use modern gypsum or thistle plaster of any sort on lime plaster.

 

[paulc]

Saw a lime plasterer working last summer, he had a pressure sprayer - one of those pump-up things for the garden - and was constantly damping both the wall and the new plaster as he wentalong.

I was going to suggest something like: http://www.screwfix.com/p/green-pressure-sprayer-7ltr/51100 - I have a 16l backpack version that I've been using to damp down my walls. Quite effective as long as you don't mind a wet floor.

As for skimming over existing lime plaster, if it is a polished surface, then a whiz over with some coarse sandpaper (60 grit) would probably be sufficient to provide a key. A cheap orbital sander would speed the job up if there is a large area to cover, but it will produce a lot of dust. A good dust mask and plenty of ventilation is essential, and the dust will kill the orbital sander after a while.

 

[Damon]

Great thanks for the replies.

I've had a look and come across the product from ty mawr called fibrelime. It seems to be perfect (if a little expensive) for our purpose.
Does anyone have any information good or bad about fibrelime or should I stick to the standard internal plaster they also sell.

Thanks for the sprayer link, I'll pick one of the up tomorrow from screw fix.

Best wishes,

 

[paulc]

At £12.00+VAT, it is about the same price that I'm paying for tubs of lime putty (except I get free delivery on the putty). The last batch of plaster I mixed up used a coarse marble powder at a 2:1 ratio of marble to lime. To the mix, a good measure of polyester fibres was added - These need to be mixed with plain water first otherwise you end up with clumps. So the process is a litre of water in a bucket, whisk in the fibres, then mix in 5l of putty to a creamy consistency, and finally stir in 7.5l of the marble dust. Leave to settle for a month or three, pour off the water, and give a good mix.

The final top coats that I'm applying are mixed from a fine marble powder and lime putty at 1:1 with a dash of linseed oil added along with some pigments. Once finished, it will be polished and waxed.

Compared to the Fibrelime, my mixes work out quite a bit more expensive and can't be applied anywhere as thick - The finishing coats are measured in tenths of a millimetre :shock:

 

[GPHouse]

Hi Hi.

I've just skimmed an old wall or 2.... I used Baumit DG27 which I got from Mike Wye.

Paints on and looks like matt white emulsion with sand in - the idea is that its breathable, controls suction and provides a key.

Seems to have worked fine with fat lime 3/2 top coat (fine) mix bunged on top as a skim layer (I did do some filling with a coarser mix first and then did a bit of scoring)....

I will use more of it, buts its not the cheapest.

 

[88v8]

Used that Ty Mawr fibre on our wall, but for the undercoat. The fibre serves the dual purpose of strengthening, and a small degree of insulation, rather like the horse hair of old.
Don't think fibre will work well in a skim.

Ivor

 

[MatthewC]

What should I do to get the skim to stay put? Is there a breathable alternative to unibond? Shall I just score the walls to provide a mechanical key?

After the scratch coat, the float coat should provide a good surface for the plaster to stay on. Use a wooden float and it comes up nice and rough, in a smooth sort of way (I know what I mean!).

I've just skimmed an old wall or 2.... I used Baumit DG27 which I got from Mike Wye.

Paints on and looks like matt white emulsion with sand in - the idea is that its breathable, controls suction and provides a key.

I used DG27 a few years ago - I understood that it was designed to give a key when you were skimming old plaster (e.g. having had to remove some areas down to the lath but left as much of the old as possible). You could then get a good skim over the old and the new with no visible join. It worked for us but it's quite expensive! My plasterer's mate painted the whole wall (unnecessarily) instead of just the old bits, and so wasted half a tub - I was pretty annoyed, to put it mildly.

Matthew