Once you have mixed your deliciously smooth finish earthen plaster, it’s time to apply it to make your cob walls shine. It’s a seemingly simple process, but there is a certain hidden finesse that will make the job easier with time and practice. I’m no expert, but here’s how I went about plastering the interior of my cob house.
(This entry is a continuation from Finish Earthen Plaster: Part 1: Materials and Recipe.)
Earth plaster tools
Your hands and a quality trowel are the two most important tools you’ll need to apply earthen plaster. I have tried using a typical American pool trowel and a Japanese trowel, and I definitely prefer the Japanese trowel, hands down.
A Japanese trowel is thinner than a pool trowel, and it has a bit of flex. The shape is also quite different. The one pictured above is the style that I used — note that the point is very useful for getting into tight corners. Below is a picture of an American pool trowel for comparison:
Pretty different. It’s nice to have a smaller sized trowel and a larger size, whether you borrow or buy ’em.
Applying earthen plaster
Wet down your walls with a pressure sprayer until they are sufficiently saturated. (I usually spray until some of the cob drips down the face of the wall.) It’s important to get a good bond between the plaster and your wall. Some folks spray a bit in advance and then come back to plaster, but I spray and plaster right away. I don’t know if it makes a difference.
Next, grab handfuls of plaster and smear them against the wall. You might choose to go pretty heavy with your material, or more lightly. I started my interior with a lighter amount, but later I went heavier. You may risk increased cracks if you apply your plaster too heavily. Do some experiments.
The initial smearing of the plaster.
In tight spaces where it is difficult to smear plaster, I would grab little handfuls and throw the material into a corner, etc. Most typically I did this in the corners of windows and around my rafters.
Now for troweling… The bigger your strokes, the more smoothly the process will go. As you glide along the wall with your trowel, you may find that you want to remove excess plaster that builds up on the steel, so slop the extra mud off into a bucket. Keep your trowel clean. Apply steady pressure to the wall, and try not to overwork the plaster. It can be tempting to try and remove every single stroke mark, but it’s not really necessary. (You can work those out with a burnisher later.) Troweling is the real art in plaster application. It’s the job that takes the most finesse and practice to master. Take your time and be conscious of your strokes, and don’t spend a lot of time going over the same area.
Burnishing the plaster
The last step in plastering your walls is burnishing the surface. Burnishing the walls will result in an even smoother surface, and will allow you to remove any marks left from troweling. Ianto Evans and company recommend waiting for the wall to reach a dryness level of “leathery hardness” in The Hand-Sculpted House, but it’s best to monitor your walls as they dry and burnish as soon as the plaster becomes dry enough to rub without disrupting it too much.
I have used both scraps of EPDM from my pond liner (which is flexible) and scrap pieces of another black plastic liner (which is much stiffer) and found that the stiffer black plastic liner works best as burnisher material for most situations. I have very briefly tried using burlap as a burnisher, but didn’t like the feel of it. If you can, try to score some scraps of black plastic liner, or be creative and try out some other suitable material.
With burnisher in hand, go over the wall in circular motions or gentle strokes to remove any lines or marks in the plaster. It takes a fair amount of time to do, so be sure to leave enough time in the day to do the job before the walls have too much opportunity to dry out.
Other earthen plaster tips
If you are doing a large plaster job that you know won’t be complete within a day, it’s important to think about how you will deal with seams. My plaster job took four days (working by myself for one, and with a friend the next three days), so at the end of each day, I sprayed the fresh plaster at the seam between it and the unplastered wall. I made sure it was wet enough so that the next day, I could continue from where I left off, and there would be no distinguishable seam.
Plastering is an art that takes time to really feel out. My experience has been pretty limited before doing my own house, so take these suggestions for what they are worth. In a future post, I will address issues of plaster cracking as it dries, which seems nearly inevitable. I have seen pretty big cracks in some earthen plaster, and in different houses, very small ones. (Thus far, I have not had large cracks in my plaster, thankfully.)
(This post is a continuation from Finish Earthen Plaster: Part 1: Materials and Recipe.)