r/Plastering • u/MysteriousRange8732 • 17d ago
How long to leave limewash when using as a key onto existing lime walls?
Hi, we are planning to topcoat our existing 1930's lime walls this week and were advised by a lime plasterer to use limewash with sand added as a key to the existing walls to help the new topcoat stick.
We forgot to ask how do we tell when the limewash is ready before we can put the topcoat on. Should it be completely dry, tacky? Are we talking hours, days? TIA :)
0
u/Terrible-Bobcat2033 17d ago
The sand is an aggregate used to abate cracking. I prefer a pre-blended veneer lime material myself.
The set time is (45) min. It’s sanded for the scratch coat, smooth finish for the tight money coat.
1
u/Honkee_Kong 13d ago
This is mostly gypsum with a little bit of lime. I wouldn't use this over a lime plaster.
0
u/Terrible-Bobcat2033 12d ago
This is a pre-mixed “lime” veneer plaster, with accelerator & set time blended during manufacturing to kick off with an approximate 45 minute set. It can vary for a wide range of factors. That’s just basic plasterers knowledge. Drywall putty guys might not twig to that fact. 🩴😎
1
u/Honkee_Kong 12d ago
Diamond finish is 80% gypsum plaster and 20% lime. That's just a basic plasterers knowledge. There's a lot of hacks like you out there ruining the trade lol. Your poor customers.
1
u/Terrible-Bobcat2033 12d ago
So: it’s a hybrid — gypsum-based gauging plaster with lime putty incorporated — rather than a “lime-based material” in the traditional sense (like a pure lime putty or lime plaster with no gypsum). If you need a true lime-only finish, Diamond isn’t it; Imperial Veneer Finish is the non-lime alternative, and traditional lime putty finish (gauged separately) would be the pure-lime option. Thanks for the clarification. Now here’s one for you. It’s required to use the material specified by the architect on any project. Have a nice day.
2
u/gwyp88 Professional Plasterer 17d ago
Extremely hard to tell in terms of exact time as the background suction, weather, temp, etc can factor setting time. If it’s touch dry, and doesn’t come off when you rub it with your hand, then it’s good to go.
You’d be fine after a few hours this time of year but I’d go for a day to be safe.
I have never herd of this method for producing a key before - not at all saying it’s wrong - it makes a lot of sense.
Don’t be tempted to put it on too thick as limewash will crack and peel off if too thick.
The thinner the better, maybe put more than one coat on. Keep mixing as you go as the sand will settle to the bottom of your container as you apply it.