Sanding A Skim Coat

[aockenden]

Hey all, we are finally making our first attempts at lime skim / finishing coat plaster internally. I’ve got a decent fine plastering sand and am doing a very lime-rich mix (about 1.25 parts sand to 1 part lime). It’s nicely creamy and moves well under the trowel.

We’re laying it on as smoothly as we’re able to, then waiting a few hours, giving it a light sponge, and then going back over it with a clean polished steel trowel. I am constantly cleaning and wetting the trowel as I go. But I’m having trouble with eliminating every last microscopic “ridge” in the finish and I’ve got limited time to dedicate to each wall. This is especially true around the curved corners of our window reveals.

My question is: has anyone had success with getting things “close enough” with the sponge and steel trowels, and then after the coat has dried, going over it by hand with a fine sandpaper to smooth out any tiny remaining ridges before final painting / lime washing?

 

[Kearn]

I definitely wouldn’t. You’ll pull the aggregate to surface and totally change the texture and appearance and also performance of the lime.

For my smooth panels, I haven’t been sponging, just stainless floating as smooth as feasible as I lay, but not overworking the lime. Then day two, with a wet stainless smaller trowel or even plasterers small tool, just smoothing back the ridges carefully with enough fat to keep them smooth, push too hard or go very and the agg starts to show.

Sponging will pull aggregate to the surface more. Sanding will wreck it.

Have you a smaller area where you could trial a few techniques/timeframes to get where you need.

 

[aockenden]

I definitely wouldn’t. You’ll pull the aggregate to surface and totally change the texture and appearance and also performance of the lime.

For my smooth panels, I haven’t been sponging, just stainless floating as smooth as feasible as I lay, but not overworking the lime. Then day two, with a wet stainless smaller trowel or even plasterers small tool, just smoothing back the ridges carefully with enough fat to keep them smooth, push too hard or go very and the agg starts to show.

Sponging will pull aggregate to the surface more. Sanding will wreck it.

Have you a smaller area where you could trial a few techniques/timeframes to get where you need.

Hmmm, yeah I could see how it would sand the top fat / pure lime off the surface and expose the aggregate. I was thinking that if I sand it first, and then slap 3-4 coats of lime wash onto it that would essentially replace the top layer of fatty lime that was lost from sanding?

I’ve been doing my finishing work 3-4 hours after it first goes onto the wall, but I’ve been wondering if that’s too soon. It’s still quite fluid / soft on the wall at that point. And now you’re saying that you do your finishing the day AFTER it first goes on? So for this next wall we’re doing today, I think I’ll wait until the next day to do my trowel finishing as it might benefit from the extra stiffness it gains overnight?

 

[Kearn]

I’m testing as I go, mostly been sponging as I’m happier with smooth, but not flat finish as many panels undulate. And it’s quicker! And as you say, once limewash/distember goes on the open surface will help adhesion and you can get a smoother look.
But on out newly built gable, with wood wool boards, everything it’s relatively flat and square, so I’ve been polishing instead. Didn’t mean to, just I got quite close unintentionally and decided to just smooth out the ridges the next day instead of sponge.

Timing depends on so much though …

 

[a twig]

Yep would agree sanding lime based plasters or renders hasn’t gone well in my experience - previously doing gypsum I’d get it reasonably good and then sand any last bits of tidying, tried the same with an NHL based render the first time I used lime and it completely wrecked the surface, ended up having to skim it all again

 

[aockenden]

Yep would agree sanding lime based plasters or renders hasn’t gone well in my experience - previously doing gypsum I’d get it reasonably good and then sand any last bits of tidying, tried the same with an NHL based render the first time I used lime and it completely wrecked the surface, ended up having to skim it all again

Wow, we’re you using a power sander or just doing some scrubbing by hand? How did it wreck the surface specifically?

 

[a twig]

Made no difference either way.

Whether powered or by hand it looked like it was actually pulling the aggregate out of the render/plaster rather than smoothing it but no idea if that was actually the case

 

[aockenden]

Made no difference either way.

Whether powered or by hand it looked like it was actually pulling the aggregate out of the render/plaster rather than smoothing it but no idea if that was actually the case

Yikes, ok I’m glad I asked. I definitely do NOT want to skim this a second time. Hopefully the coats of lime wash will blend out the small blemishes.

 

[Kearn]

Yikes, ok I’m glad I asked. I definitely do NOT want to skim this a second time. Hopefully the coats of lime wash will blend out the small blemishes.

They will, you’ve done the hard work pulling the fat up to the surface and leaving the agg buried. Any friction approach, even a sponge, will remove the fat that’s acting as a smooth filler over the top and expose agg.

 

[aockenden]

They will, you’ve done the hard work pulling the fat up to the surface and leaving the agg buried. Any friction approach, even a sponge, will remove the fat that’s acting as a smooth filler over the top and expose agg.

Yeah truth be told, I never have much luck with the sponge anyways. Which do you think is more covering of surface imperfections, lime wash or clay paint?

 

[Kearn]

Yeah truth be told, I never have much luck with the sponge anyways. Which do you think is more covering of surface imperfections, lime wash or clay paint?

Not clay - it’s bound with plastic! Will undo all your hard work…

Limewash is cheap, and you can meddle with pigmentation… but that’s whole other rabbit hole!

I’m either white lime washing, or using casein distemper from Ingilby or Rose of Jericho, as we want some deeper colours. Ingilby are much cheaper, both are great products and service. Ingilby will also colour match.

I’m unsure which would hide the sins more - line wash in theory I guess as it’s technically just bonding with the existing lime, and many praise it for filling hairline cracks. But if it’s raised parts of the surface, I’d assume both will just go over equal thickness.

 

[a twig]

Just as a point of information, the only “plastic” that is in clay paint is a Vinyl Acetate Ethylene emulsion as a binder, i.e to help stick all the other ingredients to each other and the wall. The breathability is achieved through the high clay and calcinated clay content.

So yes while not as breathable as limewash, it’s orders of magnitude better than the majority of available interior paint finishes, and is often an acceptable compromise when trading off availability, colours, durability and ease/speed of application

 

[aockenden]

Hmmm you must both be referring to a specific brand of clay paint, I’m guessing Earthborn? But of course you can even make your own with clay + water + wheat paste or linseed oil.

 

[CarolineTheChemist]

I’d agree with the Twig & Kearn regarding sanding lime plaster being a bad idea. Speaking from recent experience/mistakes it removes the smooth top layer and exposes the dusty soft sand. I covered it with lashings of thick claypaint but I only did it on a small area so I think I’ll get away with it. As I keep telling myself - “don’t make perfect the enemy of good”!

 

[aockenden]

I’d agree with the Twig & Kearn regarding sanding lime plaster being a bad idea. Speaking from recent experience/mistakes it removes the smooth top layer and exposes the dusty soft sand. I covered it with lashings of thick claypaint but I only did it on a small area so I think I’ll get away with it. As I keep telling myself - “don’t make perfect the enemy of good”!

Ok thanks, a consensus of people who have actually done it and all say how it doesn’t work well is good enough evidence for me. I’ll forgo that approach and see how the lime wash or clay paint changes the surface texture.

Yes that’s a great saying that I often come back to as well.

 

[CarolineTheChemist]

Also in my recent experiences - you only need to look at the lime plaster for it to chip, dent or scratch, so getting used to imperfections now means you won’t feel so aggrieved when it inevitably gets damaged.

 

[aockenden]

Also in my recent experiences - you only need to look at the lime plaster for it to chip, dent or scratch, so getting used to imperfections now means you won’t feel so aggrieved when it inevitably gets damaged.

Hmmm, I hope it’s more durable than all that! I thought lime is supposed to be as hard as (if not harder than) gypsum?

 

[Kearn]

Just as a point of information, the only “plastic” that is in clay paint is a Vinyl Acetate Ethylene emulsion as a binder, i.e to help stick all the other ingredients to each other and the wall. The breathability is achieved through the high clay and calcinated clay content.

So yes while not as breathable as limewash, it’s orders of magnitude better than the majority of available interior paint finishes, and is often an acceptable compromise when trading off availability, colours, durability and ease/speed of application

Yep, it’s the binder, same as an emulsion, but there often tends to be very little clay in there, except to give it some body. Don’t get me wrong, I’ve used Earthborn, and it was pleasant and had a nice solid, flat finish.

What annoys me is that clay paints are marketed as some ancient heritage product, when it’s a recent fad. And they’re not entirely honest with their spec sheets either…

 

[Kearn]

Hmmm, I hope it’s more durable than all that! I thought lime is supposed to be as hard as (if not harder than) gypsum?

That’s a very different topic!!
An NHL, (depending on brand as they all differ in terms of ingredients and the number is a lower bound, not an exact measure!), will end up harder based on some research and continues to go off for up to 10 years or so…

as with all things, hot lime based plaster can massively vary, depending on what’s in it.

The original top coats I have straight onto the daub, will literally fold in half and hang off the panel without breaking - probably about 5mm of almost pure chalk and lime, with a bit of hair. It’s incredible that after 400+ years it’s still performing.

 

[aockenden]

That’s a very different topic!!
An NHL, (depending on brand as they all differ in terms of ingredients and the number is a lower bound, not an exact measure!), will end up harder based on some research and continues to go off for up to 10 years or so…

as with all things, hot lime based plaster can massively vary, depending on what’s in it.

The original top coats I have straight onto the daub, will literally fold in half and hang off the panel without breaking - probably about 5mm of almost pure chalk and lime, with a bit of hair. It’s incredible that after 400+ years it’s still performing.

My top coat is 1 part NHL 3.5, 1 part non-hydraulic putty slaked myself from Calbux 90 quicklime, 2 parts fine sand. I’m hoping that’s going to be quite durable?