Skimming dodgy (lime?) plaster

[michaeld121]

I'm decorating my front room. When I removed the wallpaper in the bay, in places the top surface (ie no more than 1mm) of plaster came off with the wallpaper. What's left behind is sound.

The house was built in 1912 and I think the existing plaster is lime based (it's a creamy with animal hair in either the finish or undercoat plaster). I'm planning to put lining paper up & emulsion it, but when I tested it with a bit of lining paper the unevenness showed through.

Unless anyone's got a cunning solution / product I guess I'll need to skim it. I haven't skimmed any more than small patches before, but I'm prepared to give it a go - so any top tips would be very welcome.

What I don't know is if I can use modern finishing plaster on top of lime plaster? I know that the purists answer is no - but what I don't know is how dodgy it would be? If it means it would last 30 instead of

100 years - I can live with that. If it means I'm likely to have to re-do it in 6 months (bearing in mind there's been movement at some time in the bay) then I might as well do it properly.

Thanks,

Michael

[Grunff]

Yes, you can. PVA the lime plaster first to provide a dust-free surface.

The purist answer is no, and the reasons given are 'lime allows more movement and is more breatable etc'. We've skimmed our whole house, which was lime plastered, and have yet to experience any problems.

The earliest room here was skimmed 4 years ago. No crack or other problems.

[Chris Bacon]

If you can't skim it with ordinary filler (Wickes do a filler that would be OK for this) then use Thistle renovating finish. PVA the plaster before you apply the filler or finish - go around the sound surface at the edge of the area to be filled with 1:5 PVA:water. Let it dry. This will protect the sound plaster surface (slightly) from tools. Then coat the area to be filled, but do not let it dry before you apply the filler/finish (you could just damp the surface, but putting PVA/water on gives more working time). Make sure you have a nice steel float to apply the filler/finish. If you apply filler, you will probably have to rub it down a little after it dries. With finish plaster, you should have put on the finish quickly, getting it "about right", then flick some water on (or use a squeezy trigger

1L garden spray type thing) when it's firm, not soft or hard, and polish it by drawing your float across it at a slight angle a few times. Make sure you don't over-fill the damaged patches or the wall will have "bulges". You can correct odd defects in your work with filler after it's all dry. You should not need to rub down finish plaster.

[Stuart Noble]

I skimmed a room yesterday with interior filler. Used about 3 large boxes but, at £3.99 each, not a fortune. By the time you've been round looking for the bits to fill, and missing half of them, you might as well skim the whole surface. Unlike plaster, it has its own binder and is quite sound on the feathered edges. Best to wet the wall first to reduce suction. You arm will be hanging off after half an hour otherwise.

[meow2222]

yes, almost certainly lime

Skimming with lime will give you a day or so of open time, ideal for a novice plasterer. Gypsum otoh you need to work fast with, getting it right straight off.

of course, but its not the best idea. By blocking the porosity of the lime you increase the chance of damp problems later down the line. You also get little open time, which matters if youre not a skilled plasterer.

Well, I've seen it done once by a novice, and the result was crap. It needed to be redone.

BTW someone mentioned filler: filler is generally lime based.

1:2 lime putty to chalk powder (whiting) makes a good soft light fine finish. Lime putty can be made from bagged hydrated lime. Get a fresh bag, mix with water, and store it airtight for 3 weeks. Or buy.

PVA is not needed, just wet the wall and give it a few minutes to soak in. PVA, like gypsum, reduces evaporation, thus increasing the chance of damp trouble.

NT

[Chris Bacon]

Pardon? Are you suggesting that gypsum plaster is not porous?

[Stuart Noble]

He is clearly a lunatic. I wonder what he thinks of vinyl wallpaper. Now there you are talking meaningful sealing properties. You could well drown in condensation, and should hang a lifebelt on your bedroom wall.

[Chris Bacon]

I remember when he came to uk.d-i-y, and was asking about rendering, brickwork, etc. and hadn't caught Lime Disease.

[Stuart Noble]

He's been dabbling in the occult on www.periodproperty. com. I hear they dance naked round tubs of lime putty

[Chris Bacon]

Well, all sorts of DIY as well as building in general depends on compromise....

Perhaps wearing boaters stuffed with receipts for tubs of lime putty would prevent the side effects of being struck on the head by hand-made bricks of odd sizes that have fallen from sundry decaying listed edificves?

[michaeld121]

I called the British Gypsum helpline yesterday & they were definite that I shouldn't use gypsum plaster on lime.

Over the weekend I tried a small patch of Polycell Finishing Skim and it seemed to go on OK. I called Polycell yesterday & they told me that it's fine to use it on lime plaster. Does anyone know if that's correct - it just feels a bit odd (and a bit short term) to try and skim a wall with filler.

Thanks,

Michael

[Chris Bacon]

It works well as long as the lime isn't the slighest bit damp, and it helps do damp/PVA the surface. It may be the damp they're worried about, or possible use on listed buildings.

It's fine. It'll be a little harder to get a good finish for areas of damage - you may have to rub down with glasspaper on a wooden block, and apply a couple of coats as the stuff does shrink - but it's OK. Brewers and Wickes will be cheaper for a quantity of filler than Polycell.

[meow2222]

its less porous than lime, and reduces evaporation. Anyone can read up on it, but not everyone chooses to

NT

[Andrew Gabriel]

I think it's movement they're worried about. Lime plaster is normally used in lime mortar buildings where small amounts of movement are routine and don't show in lime plaster. They will show as hairline cracks in gypsum plaster. In my case, just the paint is sufficient of a filler to lose them. The other thing that can happen is that you plaster over a crack which then closes later, and the gypsum plaster blows. I had that happen once and had to replaster a 2'x1' patch. (I had made the finish coat too thin in that area, which probably didn't help.)

[Andrew Gabriel]

Gypsum is _extremely_ porous, as anyone who's ever got it wet will assure you. The difference is that lime (chalk) is substantially uneffected by moisture, whereas gypsum, being a significantly more soluable mineral, is wrecked by continuous exposure to moisture.

[Stuart Noble]

I doubt whether there's much to choose between them on the porosity stakes. I must stand a stick of chalk in water and see how far it soaks. What is obvious is that the honourable gentleman is spouting religious dogma again.

[meow2222]

Or with a bit more thought you might figure out theres more to the story. We dont always write the book on every post.

Now, must go arrange some naked dancing...

NT

[Stuart Noble]

There's a book is there? Most holy I expect. Are the rules set in limestone?