Natural lime finish on gypsum board - tips, tricks & recommendations

I've come across a few of comments on here of people doing lime finish on gypsum board. I was going to go the old lath and lime route, but decided it's just too time intensive and literally nobody makes real lath with rough edges.

I want to get as close as possible to a natural look that has good moisture control for the room as it will be for an open kitchen / living area. There will also be a Rocket mass heater in the space so lots of clay to also help with moisture control.

I've never worked with dry wall before. It will be going on a typical 2x4 (5cm x 10cm) wood framing that backs onto a 8"(20cm) brick wall. I'll be insulating on both sides of the brick.

What is the process then for this?

I see it as the following:

1) Attach the gypsum boards - what is the best kind of gypsum? are there kinds that one should avoid that leech out nasty gasses etc? How do permies view gypsum in ecological impact? Does gypsum breath?

2) Plaster between the boards, let it dry and sand it for a smooth finish. I guess this should be a rough sand job.

3) Put on a rough lime plaster with some fibrous stuff added perhaps sand and manure - How well does this lime plaster stick to the gypsum? I can't imagine it holds too well to a smooth gypsum board.

4) Finish it with a smooth lime plaster and be the envy of all kids on the street.

Is there more detailed instructions on this or if there is a more detailed how-to somewhere? I've been looking for videos but it seems most people just seem to paint onto the gypsum which I think is lame. The surfaces of the wall should have some sort of moisture absorption and release.

We'd like to get that nice smooth and bright natural looking slightly creamy colored lime finish.

Any tips greatly appreciated - thanks!

Rob

Hi Rob.

You touch a topic close to my heart.
I too will have to go this route but next year.
I basically settled on sawed lath (i'll buy raw wood and saw my own lath) because i couldn't find anything else.
Lime plaster adheres mechanically, not chemically, like cement mortars.
This is a polite way to say it does not adhere to anything.
The idea is that lime plaster has to find irregularities in the surface, like nooks and crannies to get into, solidify and then forming a key that won't let it fall off.
When it's oozing between the lath and solidifies, it's literally hanging from the lath.

The adhesion to drywall / gypsum board is very low. Let's say irrelevant.
I know people doing some "spraying" of cement + coarse sand on the drywall so lime plaster can key into it.
But that's just for walls and the lime plaster will be very thin here (2-3 mm).

In the us, they use gypsum board for ceiling that has holes in it so lime plaster can ooze thru those and key in.
See here.
I'm not  a fan of them.
Gypsum board should not have any offgassing but it is sensible to moisture.

If somehow water gets to drywall and stays a little, prepare to tear it down.
And you're doing lime plaster because it's resistant to wetting .... but using gypsum stuff will negate any advantage.

If you don't care about this aspect, ok.
Otherwise, gypsum board has good vapor absorbtion capacity.

I don't have any tips yet, but I do have a similar situation. I designed my house (a long time ago) for a plaster finish on drywall to get the hand-tooled look. I used "blueboard", which is the type of drywall designed to accept veneer plaster; however, one of my friends who was house-sitting for me decided to help and spackled all the joints and screws on a prominent part of the house. Spackle being a different and much softer material than plaster, I could no longer trust that plaster would hold on those spots, so I have determined to apply expanded metal lathing over all and proceed from there with lime plaster. (I would have needed to use the metal lath in numerous curved areas anyway.)

You mention "Plaster between the boards, let it dry and sand it for a smooth finish." I don't believe you would want to do this, but simply apply the scratch coat of lime plaster directly to the sheetrock. I would ask a professional if lime plaster adheres to sheetrock made for plastering over, to be sure. I might also do a test on a loose piece of sheetrock, and abuse it to see how it holds up. Make a corner joint as part of this test, to see how the lime plaster behaves there.

From my limited experience with lime plaster on an outdoor (roofed) cob oven, the lime doesn't seem to adhere as strongly to substrates as gypsum plaster. It keys very well, though, to a rough surface. So you might need a different substrate than sheetrock, even if it is formulated to hold gypsum plaster.


Edit: Thanks, Ionel, for your information. It sounds like expanded lath is the only way to go, even if I didn't have the spackle issue. And my feeling that the lime wasn't adhering to the cob as much as keying into it was correct.

Can you get your hands on metal lath? It's useful in creating curves but it is a little more time consuming. You'll also have to source the scratch coat (brown coat) and finish coat materials unless you're ok with mixing your own lime and sand.

Yaeh, there is also metal lath.
Here's a nice description.

It's basically perforated sheet metal or expanded mesh.
The expanded mesh is better because it has depth but also requires some spacers when fixing to the joists.
Alternatively, it could be placed directly but  the key at the joists will be less - which could lead to some cracks as the plaster thickness will be lass than between joists.

PS
If it's mild steel it has to be galvanized / Zn coated otherwise lime will rust / eat it in no time and taint the plaster in yellow.
Alternative is stainless mesh but that will put a big hole in your pocket.

For me, even galvanized expanded mesh could be too expensive.
Wood lath can be strenghthened by using fiberglass mesh.
On top and below the lath.

PS2
It seems in my area the expanded galvanized lath comes in 2 galvanizing flavors.
Chemically galvanized 1.5 year lifetime, decently priced.
Thermal Zn coating, 12 year lifetime, price is much higher ...

PS3
I did some calculations.
For me, wood lath (raw wood + cutting) + fiberglass mesh + nails is about 3 times cheaper than the cheapest expanded metal lath.
Labor is higher though.

Thanks to all for the suggestions.

After taking it all in, it really seems like although it might be quicker in the short term to go with boards, in the long run it will probably be more work as it won't last as long. With gypsum boards I'm going to have a thinner lime, without much to stick to.. and it's likely the board will get damaged before the lime. While with wooden lath, it appears the lath lasts forever and maybe a generation down the track the lime can be patched.

I have looked at both the metal and fiberglass lath and will think about them some more. I've got an order of lath on the way, but I haven't seen what it looks like yet. I think it's probably going to be pretty smooth finished and not enough keys in it for lime to grab onto. In which case fiberglass might be the way to go for lifespan as well as affordability. Using metal lath would be perfect for sculpting some nice round corners. Will keep that in mind as well.

Thanks again

If it's wooden lath you're talking abut, don't worry for the smoothness as it's not extremely important.
Just "paint" it with plain water and let it dry.
Do this 1-2times and it will form some scales which will make it very rough on the touch.
Lime plaster will love it.
You'll have to wet it before applying plaster anyway, so it does not suck all the moisture and make a weak plaster.

I plastered our bathroom with lime plaster (Tadelakt) over gypsum board.  The method I used was to apply thin-set tile mortar with a 3/16" v-notch trowel over the drywall to create tooth for the plaster to grab onto (wipe the drywall with a damp cloth first to remove dust).  The v-notch troweling was done horizontally to give the most advantage against gravity when troweling the plaster.  The plaster was applied over the thin-set mortar after it cured for at least a day.  Its been several years since the installation and there haven't been any cracks or spalling.

For earthen plaster over drywall, I brush on a wheat paste and sand mix before plastering.

One other thing i,ve seen used, mostly in the US, is gauging lime plaster with gauging plaster (gypsum).
The reasons for doing this are many.
- it has much shorter setting time because of the gypsum;
- it can stick to drywall, again, because of the gypsum;
- it's less vapor permeable than pure lime plaster which can be a plus for a ceiling;
- can be much stronger than any of them taken separately.

The gypsum to lime ratio could be anything from a scoop of gypsum to a lime plaster bucket up to 50 / 50.

Disadvantages, well, if gypsum stays wet long enough, it will disintegrate.

Hey here in Devon ive used heaps of the old laths . You might want to batten vertically to achieve a space for the haired lime to hang over underneath horizontal laths . It’s worked fine over time . I’ve also wetted the plasterboard and thrown a first coat then haired then top coat which does work too but I’d probably use stainless eml slightly battened off the backing plaster board

Interesting discussion, though I'm still wondering about limewash over conventionl, fresh gypsum plaster skim

I have a 300 year old stone cottage, with various layers of post 1950 conventional cement external pointing and interior paint. As long as there's ambient heat the building is fine, if not it gets mouldy quickly.

Anyhow for better or worse, I just gave in to government scheme to have the house interanlly insulated. They have used rock wool in a kind of woodchip/polystyrene stud frame, followed by plaster board and they are now doing a gypsum skim coat.

Bill Bradbury wrote this some yrs ago on permies https://permies.com/t/43317/Bathroom-Remodel-Retrofitting-Natural-Materials

"Gypsum is a great plaster that often gets overlooked, because that is what drywall is made from. It is still a great natural mineral plaster. I like to then apply a lime wash"

I'm aware the fresh plaster needs to dry to a pale finish and then have at least one coat of watered down paint, before a finish coat. My question is: can I use a limewash onto the fresh gypsum skim (over conventional plasterboard)?

Would I need to let the plaster dry completely first, as is the conventional route, or because I'm using limewash is slightly damp better?

And following the limewash, can I then use conventional paint (a pity I know, just loads of pots of it here when we moved in), or better to just keep up the limewash layers in the way we would do with a lime plaster wall?

Really hope someone can offer insight before the gypsum coat dries!
Best wishes from Wet and Windy West Wales, UK
Jasmine