Spraying Lime Plaster Pros and Cons – StrawBale.com

Written by Andrew Morrison

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)
plaster sprayer on straw bale wallSpraying 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.

stucco sprayerPlastering 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.)

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13 Responses

  1. How fortuitous! I had just been reading about the use of a plaster sprayer in one of my strawbale books, and was pondering the possibility… then I popped online and saw this article fresh in my RSS feeds. Thanks!

  2. Andrew, Thank you very much for answering the question! I think I’ll pick up that unit I mentioned in my original query! Actually, I was surprised that the manpower requirements for the sprayer are less than for hand plastering but as I read your explanation, it makes perfect sense. I hope to build several structures so the initial $3K investment should be justifiable. I really want to push strawbale construction here in southcentral Alaska!

    Many thanks,


  3. Having used the hopper sprayers to spray the lime scratch coat on a rather small straw bale structure, I can attest that it is much faster than hand trowelling. I had assistance at the mixer, but the application was solo. If I had to do it again (esp. on a larger structure) I would want a second person on the sprayer (as opposed to a second troweller) to allow rest periods, as hoisting that hopper over your head all day gets very fatiguing. For me (again, I was solo) it took less than one day to spray the scratch coat vs. four days to hand trowell the brown…no contest if time is an issue.

  4. Hi Andrew! Eight years ago we hand plastered our strawbale house here at Dancing Rabbit Ecovillage. On the house we tried every method we had ever heard of, including the British “harling.” It took two summers; we still have to either whitewash or put a final coat of pigment wash on the interior, but all in all it looks fabulous. We love our house!

    Last weekend, with the help of Brad Young of Fairfield, Iowa and a crew of super-human helpers, we put two coats of lime plaster on the entire two-story Milkweed Mercantile (our soon-to-open Eco Inn/Organic Cafe/Green General store). The time difference was amazing. It was not the leisurely, quiet, hand plastering experience of our first building, but wow! We found that the plaster still had to be hand “keyed” (poked into) the bales, and it was not the super-fast spray we had seen on conventional buildings. But we are thrilled with the results. We still need one more coat on the exterior, and then…on to the interior!

    Photos taken the day of the plastering can be seen here. In the week since, the plaster has whitened considerably. If anyone has any questions or is interested in visiting, please let us know!http://www.flickr.com/photos/milkweed/sets/72157603900100835/

  5. I have used the model Tirolessa hopper sprayer from the original USA supplier http://www.mortarsprayer.com– for lime and papercrete and clay plasters– it works so fast, and the best part is that you can save your energy (getting on the first two coats) then follow along and hand trowel the last layer and use your creative energy for that fun part! Also being able to stand and just spray the base/bottom of a wall rather than kneeling and having awkward arm and elbow positions to do the bottom 2 ft. is a blessing.

    * as a note– the nice cottage cheese effect of the sprayed layer is that the next layer attaches nicely to the naturally bumpy surface– something you can’t achieve with a hand trowel as easily.

  6. Charmaine, great reply! One question: are you saying that you don’t need to scratch the “scratch coat?” –that the rough surface of the sprayed plaster affords enough surface area and “grip” for the second coat?



  7. Eric,
    I strongly recommend against not scratching the first coat. A proper scratch coat, even one that is sprayed on needs to be scratched. The sprayed coat must be hand troweled into the bales after it is sprayed to ensure proper adhesion. Simply spraying it on is not enough. Once it is troweled, it will be relatively smooth and will need the scratch to make it strong enough to positively hold the brown coat.

  8. Hi Eric- I did not mean to mislead.. scratching is needed for almost any plaster system done in layers. With the papercrete plaster I was using I did not scratch the first coat, but my work is experimental, and I try many methods, and many materials and variations. Since my papercrete plaster stuck so well to itself with the bumpy “cottage-cheese” texture I considered that successful. If I were building a home and wanted long term strengh of bond I would test scratch vs non-scratch coats. Also lime plaster, or cement plaster are a different texture than papercrete, and will perform differently. This is all pioneer territory, so carefully experiment and err to the side of stronger bonds rather than speed of spraying.

  9. Hello Andrew.
    Thank you for putting together such a great resource for building. I wanted to let you know that we have just opened a new website at http://www.stuccosprayer.com . We will use it to highlight builders that use our stucco sprayer so people can see all the neat things that are being built. We started with a story on how to build a stucco fence that is made from bales. We also have an incredible arched bale cabin that I hope more people will learn about. There will be more information soon. Please suggest any specific projects that you think we should highlight.

    Best regards,

  10. Hi Alkadi. Thanks for your message. The best thing is to study the information on the website, and then check out the videos that we have in our online store. You can also consider coming to a hands on workshop for the best way to learn. Cheers.

  11. Hi Andrew wondering if u use admixtures with the lime to get it to spray also does it have to be watered down if so what about shrinking and breathing? how small is the nozzle diameter on the machine ,just wondering why it does not compress assume your using washed sand ,was under the impression lot of shrinkage or weak mortar if to wet or certain admix ,thanks Andrew be great if u could help with this.

  12. Hi Craig. When using the right kind of sprayer (a compressed air hopper and gun), you do not have to water down the mix at all nor use any additives. The lime, sand and water mix will spray just fine on its own. I don’t know the size of the nozzle I’ve used in the past. Much would depend on your particular mix; however, the plaster is not shot through the nozzle, air is. That air forces the plaster out of spray holes in the hopper. It works better than you might imagine.

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