plasterboard -> gauged lime -> lime

Woods

Woods

Active Member
trying to come up with a system to get a lime (marmorino) finish on plasterboard, quick and easy.

Priming the boards is stupid: 1. the primer costs an arm and a leg 2. I don't like painting 3. waste of time

So here's the idea: gypsum bonds really well to plasterboard so what if...
We come up with a mixture, kinda like British bonding, but with some lime mixed into it (historically called gauged lime plaster).

Then you could just skim coat the boards with a 3-5mm coat of sticky gauged lime bonding coat, flatten/rule/scour and as it's starting to firm up, give it a second coat of fine lime (marmorino) top coat. Done.

To possibly improve the bond, one could use a paper tiger tool thingy to score the boards. This exposes some of the gypsum so potentially this could improve the bond with the gauging/bonding plaster even more...

plasterboard -> gauged lime -> lime


Thoughts, opinions, rotten veg you want to toss at me?
 
soft like Pavatex or cement based and hard like Cewood? Both systems are more expensive. How do you even attach the softer type boards like Pavatex? You need those big plastic *screws* for them?
You also probably can't plaster it without fiberglass mesh? Another significant added cost...

Drying times are pretty slow on those boards too; so you can't do a same day finish on them...

Going the purist route on a ceiling doesn't make sense to me. Pure lime and clay plaster only makes sense in vapour permeable exterior wall assemblies. For a ceiling, it doesn't matter if it has gypsum in it, imo. Ceilings have very little (if any) effect on the ability of a building's wall assembly to dry out efficiently, which is one of the core reasons why lime and clay based plasters are so useful.

For a 30m2 ceiling with plasterboard:
100€ for the boards
20€ for the screws

Same ceiling with wood wool boards:
365€ for the boards
probably 100€+ for those big plastic screws
50€ for the fiberglass mesh
and you can't finish it in one day
 
a lot of folks seem to have doubts about that. While I have found the lime bonds pretty well to the boards, theoretically, it's not a kosher bond. Gypsum actually chemically bonds with gypsumboard. Then there's different rates of hydrothermal expansion between lime and gypsum. So to me it seems prudent to go with a basecoat that has a lot of gypsum in it, but also enough lime so that the final layer can be pure lime. That way you have somewhat of a buffer between the pure lime coat and the gypsum board.
 
did some more experimentation:
0.5 part quicklime powder
let it slake with a little water (5min)
then add 2 parts sand and 1 part gypsum
mix well

Application of this stuff to the gypsumboard ceiling is the hardest part; it's not super sticky this mix. So it's a bit messy. Might be better to use only 1 sand perhaps.

The trick is to apply it, flatten and rule it straight away to get it within reason flat (later on we're going to scour it, so it does not need to be perfect, it just needs to be level; little hollows are fine here and there) but then the important thing: leave it alone for 2 hours.

After 2 hours the stuff is really firm and sticks like glue to the ceiling but if you hit it with your spray bottle it scours lovely: hollows are filled very quickly, a nice even and smooth surface is obtained rapidly. I think the scouring leaves fine enough of a surface to come back later with the marmorino, but I suppose you could trowel it up right after scouring. Do 1m2 of scouring, then quick polish to embed the grains and then scour the next 1m2.

So far I'm pleasantly surprised. Adding gypsum makes the lime stucco adhere a lot better to the board. Drying time is also more manageable. Lime only would take 5 hours to dry enough to the point where you can start scouring. So it's too long to do an entire ceiling in one day because scouring also takes a lot of time. Scouring the next day is usually too late. But with the gypsum added, you can scour with a good amount of water in 2 hours. So if it takes 2 hour to put on a 3-4mm coat on a large ceiling and flatten it, by the time you're done, you can start scouring where you started and work your way slowly to where you ended. So this give the ability to do the base coat on your own comfortably on a large ceiling.

It's best to leave the base coat to set for 24-48 hours. The gypsum will have gone off totally and stabilised. But the lime is still in the process of curing. I think it makes sense to only apply the 1mm fine lime finish (marmorino) when the gypsum is stable but when the lime is still curing. Then by apply the finish coat, you introduce some more moisture into the basecoat so that the lime in both coats can carbonate in unison, which is probably the most ideal scenario.

If further testing is successful I'll soon prep to do a 30m2 ceiling and will share my findings
 
okay so first test was not bad in terms of bonding to the gypsum board, but some improvements have been made ad interim:

first coat: 0.5 quicklime : 1 gypsum : 1 fine sand
second coat: marmorino
done

Large ceiling can be finished in one day. Tried to get the plaster to delaminate from the boards brute forcing a putty knife into it, but it won't come off. This is actually problematic because now my test patches are stuck... So I'll have to tie them in somehow when I do the entire ceiling. I suppose I didn't anticipate it would bond that well... oh well...

btw, no primer or bonding agent was used. Just plaster straight on the gypsum board. And mind you, this is gypsum board intended for drywalling (tape and mud) not for plastering.

It is important not to overwork the first layer though. If you mess with it for too long, you push too much of the larger aggregates against the board, weakening the bond. But if you just apply it, leave it, and flatten at the right time, you get a very strong bond.
 
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interesting discovery:
my carbon steel MT does not rust when I lime plaster with it.... and no matter what I do to the metal, it refuses to sharpen.
After doing this gypsum test on the ceiling though, a few weeks later and my trowel turned into a pile of rust and the edges felt sharper than they usually do.

So I wondered if gypsum reacts with carbon steel and surely it does:

plasterboard -> gauged lime -> lime
 
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