Scouring lime plaster finish coat - WHEN?

[jbaldwin]

I'm in the middle of putting on a lime plaster finish coat. Its on the stone walls either side of a window seat, and the walls have got some fine undulations in them that I've deliberately not flattened.

I've put the plaster on with a trowel and then rubbed it into the shape I want with a combination of a cross-grained wood float and, for the concave areas, a smooth rubber grouting float. This has worked very well. It's the next bit I'm unsure about. The finish at the moment is very sandy. I'm told that after "a few hours" or "tomorrow" or "when it's gone off a bit" I should scour it with a wooden float and a smoother finish will appear as if by magic. Is this right? And how set should it be and how long can I leave it? I'm worried that if I leave it too long I'll be left with a too-sandy finish, but I know if I do it too soon (I've just tried a bit) it just takes it of.

 

[Ian Bond]

If you want the finish smooth, use a steel float and apply it in two thin coats.

Or use the steel float after scouring with the wooden float.

A wooden float alone will leave a slightly open grained texture. This is no bad thing if you are going to limewash the wall - it will be a good base for the wash.

And a few coats of limewash will smooth the finish too.

As you will have noticed, the finish coat will harden quite quickly. But try not to overwork it or polish it too much.

It's just a matter of practice, trial and error. You will find the technique that works best for you and the finish you are after.

But it is harder to do than the scratch coats!!!!

 

[jbaldwin]

I do want an open texture because the wall needs to breath, and anyway its an old cottage.

So I'm going to scour it with wooden float, using grouting float in the concave bits. But WHEN do you do this? How set does it need to be? I've just tried a bit, about six hours after it went on - seems to be rubbing too much off. Hmmm. Perhaps leave it till the morning.

 

[Ian Bond]

Oh and *light* spray a of water onto the wall as you polish, but again, not too much.

 

[Ian Bond]

When is a very good question - you will have to 'suck it and see'.

Depends on the mix of the finish coat, how much suction there is from the background, how well you wet the wall, the ambient temperature, how thick you apply the finish, and so on.

As you've noticed, if the plaster is too wet when you try and rub it with the float, it sticks to it and comes off. But then if you leave it too long, it becomes extremely difficult to rub it to a good finish - you can try spraying a little water on it.

It because of these difficulties that I tend to use the method that I find slightly easier. I apply a thin skim, and then straight away apply a second coat over it, using a steel float. This saves me from having to scour the finish with a wooden float, and from having to guess when it will be ready for me to do that.

I've also found less crazing of the finish with this method.

However the method you are using is just as valid, and all the better if it allows you to closely match the original plaster in your house.

Hope it goes well,

Ian

 

[biffvernon]

Does anyone have an explanation as to why a wooden float gives a rough finish and a steel float a smooth surface?

 

[Ian Bond]

Well the steel float has a smoother surface, and bits of plaster don't stick to it as well as they seem to stick to my wooden float.

The steel float also seems to 'suck' the lime to the surface, making it smooth.

The wooden float doesn't do this, well not for me anyway.

I suspect that the grainier finish given by the wooden float is more breathable, and better to accept limewash, too. And probably the authentic way of doing it - when did steel floats come in?

Has anyone here managed to get a very smooth finish just using the wooden floats? What's the secret?

 

[Alastair]

The steel float not only has a smoother surface, it is airtight, so it has a tendency to hold water under it, also holding the finer particles near the surface since they're floating about in the water. Wooden ones have leaky surfaces and so don't draw the water up.

 

[Mark]

Has anyone here managed to get a very smooth finish just using the wooden floats? What's the secret?

No.


But Im going to try with a cloth polish next time - I remember a plasterer polishing with a bit of muslin.

I've managed a moderate finish with a cheap sponge.

 

[Gervase]

You can get two types of sponge float - one with a fairly coarse texture which gives a nice scoured finish and one with a fine texture (like the inside of a wetsuit) which gives a much finer finish. If you're having problems with the wooden float, try the fine-grade sponge; it's ideal for evening out the surface and ironing out the odd awkward line and scrape if used with a water spray when the skim has had a couple of hours to firm up. For a really polished surface, though, a steel float has to be the best way. And, if you want a super-fine marble finish, you can then burnish the dry surface with a lump of soap. I'm sorely tempted to try that with some of Ty Mawr's 'glaster' (which has some lovely, jewel-like fragments of green and blue glass) when I redo the bloody bathroom walls after having been persuaded by the management to put in a wainscot and a bigger shower.