Ready to go full-on with your plastering project, only to find out you canÃ¢â‚¬â„¢t get any information on historic lime-based plaster? Welcome to my world.
I finally tracked down an old encyclopedia that had good basic information, and I augmented that information with some good old book learninÃ¢â‚¬â„¢ from my graduate program (in preservation) and a preservation trades conference or two, and here we are.
A lime-based plaster with sand and hair (generally 2:1 sand:lime, 3:1 sand:lime, or somewhere in between) is the mix for most historic homes. You can get a rough guesstimate of what your particular plaster mix is by weighing a chunk of mortar, putting it into a bowl, using vinegar to dissolve the lime, and screening out the sand. Weigh the sand once it's dry and you'll know about how much weight sand to the weight of the lime. Like it said, it's rough, but better than nothing. You can also get a lab to test your plaster and give you an exact detail of what you're dealing with.
I bought Type S Hydrated Lime at my local masonry builders' supply. I took a 5-gallon bucket, filled it about a third of the way with water, then began adding lime and stirring until it was all well mixed. I continued to add lime and to mix well by hand until my putty had the consistency of a thick yogurt. Then I covered the whole shebang with about an inch of water so the lime wouldn't begin to carbonate, and let it sit for a while. Funny thing about lime putty, it's actually better the longer it gets to sit. So be patient if you can.
I used general-purpose medium builder's sand--ideally you want a good "sharp" sand because sharp sand holds onto the lime putty better (extra surface area is a good thing!). I did NOT use any gypsum ("guaging plaster") because it does not hold up well in a moist environment.
Mixing the plaster went as follows:
--Slop out some lime putty (keep track by volume of what you're using) into a mixing tub
--Add sand: my encyclopedia suggested using a mix of one part putty to three parts sand, plus hair, for "coarse stuff," one part putty to three parts sand for the second coat, and putty alone or one part putty to one part fine sharp sand for the skim coat.
--"Knock it up" by mixing, beating, mixing, beating, mixing again and it will become more and more plasticized. This is EXHAUSTING but makes all the difference in the world. Just when you think it's never going to come together and be workable, it will. Keep at it.
--Add the hair (for plaster only). The amount of hair is kind of a judgement call. I added and mixed until it seemed like things were fairly "hairy" and then put some on a trowel and hit the trowel sharply against my bucket. I ended up with about a 5-inch glob of plaster hanging to the trowel, and there were lots of hairs visible hanging down. A fabulous, fabulous session at the Traditional Building Conference taught me that little rule of thumb...er, hair. My old encyclopedia notes that the second plaster coat can be mixed minus the hair or with the hair in halved amounts, and straight lime putty or one part putty to one part sand for the final coat.
--After I had what I thought was a reasonable mix, and after thoroughly wetting everything and spraying with a bonding agent, I started troweling it onto the walls. I have no shame when I say it's HARD to get traditional lime plaster nice and even. HARD.
In applying plaster, I once again consulted my trusty ancient encyclopedia, which also mentioned the after-care of plaster. This included misting (did that) as well as scouring with a wood float. When I first tried to scour the plaster, it was not quite cured enough, and some of it stuck to the wood float and came off. This, obviously, was not the intended result, so I stopped. But I did mist. Scouts' honor!
Now my beloved skim coat that took so long is cracking here and there, despite the misting. I figured I had nothing to lose by trying to scour it with the wood float since it has cured for a couple of days now. With more than a little trepidation, I took said float in hand, wet it down, and began scrubbing my sweet, innocent plaster wall in a circular motion. Hard. Holy moley! Everything kind of evened out, the cracks disappeared (with a few exceptions . . . feel my pain below), and my wobbly uneven skim coat began to look like REAL plaster that had been done by someone who knew what he/she was doing. Do you believe in miracles? Yes!
More about the wood float: I was a real nonbeliever at first. I mean, who seriously would think that going back to the plaster (the plaster you labored so hard at to just get to stay on the danged wall) and ripping back over it with a piece of wood could somehow magically make it smooth and crack-free? But somehow it does work. My old-timey plastering guide notes that scouring with the wood float is a necessary step on the process because it "gets the fat back." While I'm not hip to my 1920s-era plastering terminology, I think what it boils down to is this. The skim coat is made of lime putty and sand. Steel finishing trowels have a tendency to (I don't know how or why) separate the two and cause a less-than-strong finished product. The source I have says to go over it once with the steel trowel, just to get it to a modicum of smoothness, then leave it alone. I had a verrrrry difficult time trusting in this logic. After all, wasn't I striving for that beautiful, smooth, clean plaster wall? But by gosh, it actually worked. I put up the skim coat, troweled it over once, and left it.
"Getting the fat back" refers to the wood float's magical ability to push the lime and sand back together and compress the whole shebang into a lovely skim coat. Before I used the float, my walls looked....well, kind of sandy/plastery, not like smooth plaster should look. But after I used the float, my walls looked super smooth after I was finished. One note of warning: scouring with the wood float is a killer on the wrists! You have to try to maintain an even pressure, all the while scrubbing in circles, but also trying to not create any suction between the float and the plaster.
Apparently people who have oodles more experience than me with plaster can actually "polish" a wall to practically a mirror finish using a float. I don't envision that being in my repertoire anytime soon, although I'd sure like to try my hand at it. Virginia Limeworks has a finish that they use in bathroom environments that actually looks almost like a perfect white piece of marble, minus the graining and feathering. It's that smooth and shiny. In person, it's absolutely drop-dead gorgeous.
So there you have it. Plastering! Woohoo!