When it’s time to plaster your cob walls you will want to make use of finer materials. Rocks, big pieces of straw and the like can really screw up the finish, that is if you are going for a smooth surface. Recently I have started buying the quartz silica sand from my local home center for $2.50+ a 50 lb. bag. The silica sand is a pleasure to work with. Fine materials can create fine results. $2.50 a bag can add up quickly for a large project.
I’ve been mixing 3 sand with 1 lime putty for a lime plaster. There are some small gaps and holes in the walls surface that needs filling. I sure don’t want to fill them with the expensive silica sand, and I don’t really want to go back to adding cob to the wall either. I decided to start with a pre finish / scratch coat with the ‘cheap’ sand and lime.
The sand pile that I have on site isn’t very good for the finish coat, with the rocks and all. It’s great for cob, with small to medium sized pebbles andangular little rocks. For my plastering purposes I send it through my fancy homemade sifter. Of course this saves money, but adds to time and labor. There’s also something to be said about making use of what you’ve got on hand.
I created a couple sieves. The first is meant to filter the larger rocks ( 1 cm + ). The second is for the really fine particles. I made them from a couple of 2 x 4’s, some screws, the screen and some plumbers tape – not too high tech.
When it’s time to sift. I place the wheelbarrow by the pile and toss in a few shovels of sand. Then I shake and agitate the material until the sand falls through the sieve. I’ve found it’s nice to use a rock that fits the shape of your hand to slide across the screen; this can make quick work of it. A few shovels full and you have 100 lb. of fine sand.