I work almost exclusively with lime plaster. My favorite is Natural Hydraulic Lime from St. Astier (www.limes.us). I teach about how to properly wet your plaster and one detail that often comes as a surprise is the amount of water that is needed to complete a good plastering job. The obvious water is that which is used in the mix; however, that is only a minor slice of the pie when it comes to the necessary water, especially if you live in a hot/dry climate.
Follow these steps to make sure you have the best plaster job available. Be sure to protect your walls from wind, rain, and direct sun by hanging tarps. There’s more to that, but the scope of this blog post is about water, so let’s address that directly.
Wet the straw bales before you plaster. This freaks a lot of people out during my workshops. They hear over and over again how important it is to keep the bales dry and then I spray them (the bales, not the people…okay, sometimes I spray them too!) with a hose before we plaster. It is an important step to take and without it, your plaster won’t properly bond to the bales. It takes quite a bit of water too. You want the bales damp, not running with water, nor slightly misted. We are looking for damp.
Mist the walls for at least three days after both the scratch (first) coat and brown (second) coat. This helps to slow the curing process down and strengthen the plaster. The added water allows the lime crystals to grow at a slow and steady pace while still having a ductile sub straight to move through. I use a gentle mist the first day (as soon as the wall starts to look dry) and then increase the amount of water I use each time. If you add too much water too soon, you can wash the lime off the wall. Be careful, but remember that the goal is to keep the wall damp, once again.
Soak the walls between coats. There is a 10 day cure period between plaster coats when working with lime. The requirement for keeping walls damp is only 3 days, so the last week will likely see the walls dried out. If you add the next coat of plaster to these dry walls, your plaster will not adhere properly as the dry coat will suck all of the moisture out of the fresh coat. To avoid this, be sure to completely wet your walls the night before you plaster. This takes a lot of water. You will know when you are done when you can spray the top of the wall and the water actually runs all the way to the bottom of the wall. If it doesn’t make it down, the wall is still too dry.
Spray the wall again the morning of your plaster job. You want the walls to be damp, but not running with water.
Repeat steps 2-4 for both the scratch and brown coats. When you get to the finish coat, everything is still the same except that you don’t mist the walls AFTER you plaster. It is crucial that you soak the walls prior to plastering as outlined above.
There is far more water used in the preparation and curing process than in the mix itself. If you have a limited water supply, be sure to account for this extra water requirement.