Many people have asked me if spraying lime plaster is a good way to go. There are belief structures on either side of the coin. Some people believe that spraying the plaster will improve the adhesion of the plaster to the bales because the plaster is being forced, via compressed air, into the bales. Others believe that hand application provides the best adhesion because the plaster is pushed and shimmied into the bales with arm power. There are other pieces of the puzzle to consider too. How many people do you have available who know how to plaster? How long do you have to get the plaster applied? What is the weather like? Do you have access to a commercial grade plaster spraying machine? Will a small hopper style sprayer work for you?
(photo courtesy of Harvest Build)
Spraying plaster will certainly adhere the material to the bales as well if not better as hand application. The force at which the plaster is applied is enough to inject it into the nooks and crannies of the bales. Furthermore, the sprayed application must then be troweled in by hand anyway, so you get the push of the arm power as well. If you don’t hand trowel the sprayed application, you will not get a smooth surface and the adhesion will not be as good. It certainly speeds up the process to spray the plaster on. The initial application is much faster with this method. Be sure not to spray the plaster too thick though. If you spray on the plaster thicker than the approved depth, you risk increasing cracking in the finish coat and those cracks are more likely to telescope all the way through to the bales.
Plastering with a sprayer allows you to minimize the number of people you need on site. Being that the plaster is sprayed on quickly, you can get away with say 4 people to apply the mud. One would be married to the mixer, one would be in charge of spraying and two would be the trowel hands. That is a good number of folks to work with as hand application can require up to a dozen people on a 2000+ SF house. The speed at which you have to mix and/or spray the plaster will depend on the sprayer. Commercial sprayers have a big hose that pull from a remote location. The amount of mud sprayed is high and the mixer will have to start earlier than the spraying crew to create enough mud to keep up. Smaller units are available like the one above that can speed the process while not speeding things up “too much.” They are also much less expensive to get and can be purchased by the average owner builder. You can purchase one at www.LimeWorks.us for $250. You will also need a compressor big enough to run the sprayer. (If you decide to buy one, contact me first as I can give you a vendor code for a discount. This is true for your lime as well.)
About the Author
Andrew Morison is a specialist in straw bale and green construction. He has shown thousands of people how to build their own straw bale projects through his comprehensive series of instructional straw bale, concrete foundation, and plastering DVDs, as well as his hands on workshops. You can check these out at www.StrawBale.com/store.
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