Chamfered Lime plaster corners

[housegolem]

Hi All

Im hoping the forum can further help me in my pursuit of a perfectly plumb lime wall. I've found a lot of useful information on here but now have a specific question about lime plaster corners without any reinforcement.

I've been re-plastering a fair amount of my 1920's house, skims on lathe stud walls, solid walls skims, full three coat work and patching, so have a pretty decent grasp of the challenges and pitfall of plastering with lime. I've been mixing all my coats and steeping them in wheelie bins!

I've reworked a couple of fireplaces already (picture below) but now have a large chimney breast downstairs which is missing some of the corner edges. The skilled plaster before me has finished the edge with a chamfer and im pretty sure he has used a slightly different (harder) mix on the corners for added strength. What i want to know is what is in this mix ?

There's a comparison picture, with a small piece of it in my hand with a larger piece of standard stuff. It's slightly more grey in colour. There is also a cross-section picture with the softer plaster below the harder grey pre skim.

any ideas?

Many Thanks

 

[malcolm]

We're probably into nobody knows territory here. You can get harder limes buying pre-mix NHL stuff where NHL5 is as close to cement as you want to get.

For a fireplace opening that is going to be used a brick opening is probably best. I've gone for a curved edge on the inside corner where the front of the fireplace has to be plastered.

 

[paulc]

The corners on most of my walls (late 1920s build) appear to be made of cement. They are very hard, and the one that had to be removed (over the bay window and had cracked) was heavy. It is probably a regional thing like most construction methods & materials.
Where I've been reskimming the walls, I've used modern plastic corner beads.

 

[housegolem]

Thanks for the replies

From the top

Malcolm - Ive got some 3.5NHL that i used on the chimney so that's a possible solution. I could run a couple screeds up the sides and then infill with my putty scratch mix.
The fireplace will be used and has brick all around, so on the front face im just going to blend in with a sponge like i did on the upstairs ones. I'm more worried about external corners with impacts, whatever they did before has weathered pretty well so was hoping to copy that.

Paul - It could be cement, it's definitely grey in colour and hard. I just can't get my head around how that would be applied as it doesn't go down to the brick. I will break off some more an take a closer look, im in Berkshire so down south. Out of interest how well does the lime stick to the plastic beading?

What i do know is i'd have a beast of arm if i could two coat the entire house to a super smooth finish using 0.5 -1.5mm of top coat

 

[Lacan07]

Thanks for the replies

From the top

Malcolm - Ive got some 3.5NHL that i used on the chimney so that's a possible solution. I could run a couple screeds up the sides and then infill with my putty scratch mix.
The fireplace will be used and has brick all around, so on the front face im just going to blend in with a sponge like i did on the upstairs ones. I'm more worried about external corners with impacts, whatever they did before has weathered pretty well so was hoping to copy that.

Paul - It could be cement, it's definitely grey in colour and hard. I just can't get my head around how that would be applied as it doesn't go down to the brick. I will break off some more an take a closer look, im in Berkshire so down south. Out of interest how well does the lime stick to the plastic beading?

What i do know is i'd have a beast of arm if i could two coat the entire house to a super smooth finish using 0.5 -1.5mm of top coatView attachment 12957

Yeah, its incredible really, there isn't nearly enough material there to float and then flatten. They must have flattened as they applied, how did they get it so flat! I reckon they must have mixed it pretty wet, not so much like the buttery textured skim coat lime you see today.

The more I learn about lime the more I appreciate the people who plastered my house, and the more annoyed I get at the fact I was conned into removing all of it downstairs. I mean I would have insulated anyway but it would have been nice to have the original plaster as the base.

I've got some external corners to do on my chimneys and I'm not looking forward to it. I'm just not confident enough to try rounded corners freehand. I'm thinking of opting for the P section wooden beads.

 

[housegolem]

I think they did the majority of the flattening on the first coat with a lot of heavy scouring. How they did the skim i don't know, i think their sand was a lot finer for a start than mine and possibly a much richer mix of lime putty too.

As for a rounded corner i would be very wary if you are after a polished finished but not so much so if you are going for a sponge floated/grained finish. The majority of the shape would be come from your lower float coat, so as long as you built up a descent thickness of top coat you would struggle to wear through it during the finishing stages.

 

[paulc]

Paul - It could be cement, it's definitely grey in colour and hard. I just can't get my head around how that would be applied as it doesn't go down to the brick. I will break off some more an take a closer look, im in Berkshire so down south. Out of interest how well does the lime stick to the plastic beading?

Could be a precast angle bedded in on lime.
As for plastic corner beads, they have a multitude of holes so that the plaster grips the underlying surface. Even so, I found the plaster I was using gripped quite well to the plastic.

 

[Stevers]

Both homes that I've owned had round wooden staff beads of about 1" diameter in all the external corners. Generally there is a groove where the plaster ends that you would trim the wall paper into. Historically these were painted to match the rest of the woodwork, but previous owners had filled the grooves and papered round the corners. Needless to say we cleaned the grooves out and made the staff beads a feature - apart from in our basement where there were never any grooves and the plaster and staff beads were painted as one.

 

[Lacan07]

I think they did the majority of the flattening on the first coat with a lot of heavy scouring. How they did the skim i don't know, i think their sand was a lot finer for a start than mine and possibly a much richer mix of lime putty too.

As for a rounded corner i would be very wary if you are after a polished finished but not so much so if you are going for a sponge floated/grained finish. The majority of the shape would be come from your lower float coat, so as long as you built up a descent thickness of top coat you would struggle to wear through it during the finishing stages.

Thanks, I'll be going for a float finish probably, I think a polished finish is beyond me perhaps. I don't know how the 'flatten as you go method' is supposed to work, maybe I don't have the strength. I tend to apply 2 skim coats one after the other and then float flat. In theory I could then flatten but the street lights are usually coming on and I'm knackered. I also think it would be better with 2 people on the job - one to float up and the other to go in with a trowel to flatten. To be honest I have really experimented much with polishing lime putty, maybe I ought to.

This is what I'm thinking of using for the external corners:

 

[housegolem]

The type of wall and what preparations you've made before hand make a huge difference to the time available for you to work and finish the wall. The first time i did a large wall from bare brick it was two days before i could finally finish working on the skim coat! This was because i had been tending each new layer regularly over several weeks and then applied three skim coats. The lowest point of the wall being the slowest to dry as the water drained downward.

In comparison i've skimmed over smaller sections of original scratched lime surface and finished in half a day getting a similar quality finish.

Putty is quite forgiving and a long as its wet you can work it, i've often left walls overnight and continued the next morning. It's a balance between slowing it enough so you can work at your current skill level, and then not having to wait days for it to set!


I've had a go with the wooden float and agree its very hard work and also difficult to time, i also didn't like the staining that came off the float.

So have created my own method.

One coat of skim 2:1 - sometimes another if i have misjudged the depth or want a really flat wall.
Then a 1:1 skim, i let this dry out until the surface can move put won't pull when i go over the whole lot with a plastic float. Usually starting at the top of the wall (any area that dries quickest) and going in a circular motion. I use this to almost plane and scour the surface. Any highs that are knocked off i push back into the highlighted lows.
I leave this to settle and dry again
Then i hit the wall with a sponge float loaded with skim and smooth over with a steel trowel as i go.
Like all plastering timings are important! by going too early i've had delamination and the dreaded blebs (bubbles)


The finish is smooth , close grained rather than a true polish in the sense a gypsum wall would be.

 

[paulc]

I think a polished finish is beyond me perhaps.

It isn't that difficult -

 

[housegolem]

good clickbait!

 

[Lacan07]

It isn't that difficult -

I love the way his trowel is brand new for each coat and there isn't a speck of plaster anywhere in the room. You should see my rooms after a coat, it looks like a bomb's hit it.

 

[Woods]

how did they get it so flat!

you have to make screeds, that's the secret.

One dot above, one dot below. Let it pull in a bit until it feels stiff but still malleable. Take your hand float and scour both dots. Then take your plumb bob or level to see which dot needs more scouring. Repeat until the two dots are perfectly plumb. Then connect the dots with a line of plaster. Let it pull in a bit. Then take your rule/darby and float the entire screed plumb and flat using your two dots as reference points. Repeat this entire process to make multiple vertical screeds on your wall. Make sure they are spaced out as such that your rule can glide on them.

Then you just fill in the areas between the screeds and you use your rule to float it all nice and even as you go. Any hollows, fill them up and then float with the rule once more. Let the plaster pull in a bit more and stiffen up but still malleable. Then re-wet the entire wall and start scouring. When you're done, wait a little bit and then scour once more with less water. Then a final time without water. You should now have a perfectly plumb, straight, flat and smooth surface.

While the plaster is still green but firm to the touch, apply your putty coat (very fine sand and lime 1:1'ish) and apply a couple mm thick. Allow it to pull in a bit, but don't wait too long! When the plaster is ready, take your clean hand float and start scouring to obtain a flat surface. You can use water, but sometimes it also works without. It kind of depends on your personal technique and the plaster recipe. Immediately after scouring, wet the plaster again and now trowel it up. Work the fat from the trowel back into wall to fill in all the small pores between the sand aggregates to achieve a smooth, glass like finish with a gentle, extremely fine texture. Then trowel it once more without water. Don't be tempted to over trowel or the plaster might peel of the wall. When you see bubbles, don't worry about them.

There are many ways to get a good result, but this is my personal favourite because it is very economical and fast but you get a lovely, smooth and even finish. And if you screeded properly, your walls will be perfectly plumb/flat within 1mm tolerance. Of course it takes some practice to get to that level of accuracy but it's possible. Just take your time when you're learning, do a lot of testing, be patient and one day it'll just "click".

 

[Janela]

There is also beauty in a flat, smooth plaster that follows the contours/undulations of the wall itself. A bit more rustic but that is what I've done on our 14thC rubble limestone walls.

 

[Woods]

Plastering an organic wall is an art onto itself but I personally don't like it at much in certain light though I'm sure in some design contexts it can be very appropriate!