Benefits of Spraying Plaster on a Straw Bale Build

Written by Andrew Morrison

old Scottish castle

I can’t believe it, but I have been converted! I never thought I would stand on the side of spraying plaster as I have always believed that hand troweling is better; however, I was convinced of it this last week. After talking with Steven, a master lime plasterer from Scotland, I am fully convinced that there are many benefits of spraying plaster.

As you may know, Scotland is known for having very old buildings, castles like the one above even, that have been plastered and mortared with lime for centuries. The knowledge that Steven shared with us has been passed down for generations and is something I could easily understand through his teaching.


Here’s the deal, when sprayed on to the wall surface, the lime is compacted as it hits the wall. This compaction works in a way similar to pressing back as described in yesterday’s blog. In addition, the force of the impact of the spray helps to drive air out of the lime. On the other hand, when troweled on, the action can actually put air into the space between the substrate and new coat. This can lead to sheeting and plaster failure.

Traditionally, plaster was cast on to the wall. This means literally throwing the plaster on to the wall. For the same reasons as described above, this method has endured for years. There is a skill to a proper cast, so the idea of a machine that can do it for you and at a higher rate of application is a great idea. In fact, Steven says he would use it for all new applications, just not for historic work where the art of hand casting is still the first choice.

Spraying the wall and following the sprayer with a trowel to push back the plaster and create an even coat produces a strong plaster and a strong bond. You can actually leave the plaster with the cast texture if done properly which creates the necessary key for the brown coat and increases the surface area of the wall. This is great because it aids in the carbonation process by exposing more of the wall to the air. If you cannot leave the texture in place, be sure to press back the plaster and then scratch it to expose more surface area to the air for carbonation and to provide key for the brown coat.

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34 Responses

  1. Andrew,

    Thanks so much for this info!! =) Do you mix it with the morter mixer first? Then how do you spray it on? And can this be done by us or do we need professionals?

    thanks, Tamra

  2. Good questions. You mix the mud the same way you would if hand troweling. be sure not to add too much water which will weaken the plaster. You can buy a gun at under the tools and training page. In fact, you can use my vendor code: 95501NHL and get a discount on the sprayer or anything else you order from them. You don’t need a professional to do it, you can do it yourself.

  3. Wow – we should all be so lucky as to learn from a master from Scotland! Way cool, and very interesting historical stuff, too. 🙂

  4. I am on my third coat of lime plaster interior and exterior. I hand troweled the first two, can I spray on the third coat?

  5. You can spray on the third coat. It is similar to a cast on finish. If you want it smoothed out, you can trowel after spraying. Be careful not to over work it.

  6. Will you be making a new plastering dvd showing the spray technique now that you have learned this??

  7. Hmmm. Good question. I had not planned on it. The plastering DVD I have already has a LOT of really good information that is not destroyed with this new information. Spraying is the best way to apply, but troweling can be done with amazing success as well.

  8. So do you spray the lime on as a first coat, directly onto the straw bales or can you put on an earthen coat first, to fill in the rough places and make things more smooth for the lime (and also not cost as much)? Would the lime adhere to an earthen first coat?

  9. I would not use a first coat of earthen plaster at this time as I have heard of a lot of problems arising from that practice. I personally believe it can be done, but the risks of making a mistake are large and if the process is not done perfectly, your entire plastering project could be bust. I don’t see an issue using the earth to fill in small depressions, but not smoothing the entire wall. Stick with the lime. The cost will pay for itself in the end with a quality finish.


  10. We have applied lime over earth with varying results. On one wall of a two-building project, the lime plaster did not adhere very well to the earthen brown coat. If you think about it, there is no chemical reaction taking place to create a bond. Intuitively it does not seem like a good idea, and after my experiences I would say, if you want to do it, use caution and do some homework.

  11. Hi Andrew, I would like to also ask you to make a spraying plaster DVD. Some of us are thousands of miles away from you and have day jobs that we must maintain. Please, please!! See you in new mexico!! …I’ll open the hot tub for our workshop if you do!! please…

  12. Maybe someday Charles! I have a lot on the plate right now and so does my videographer (my wife). 🙂 Could be a fun project though.

  13. Hi Andrew and fellow straw balers,
    We have just spent three days spraying the first coat of lime plaster on our project near Klickitat, Wa.
    We used the sprayer that Andrew has suggested and give the whole process a big thumb’s up!
    This is our second Strawbale and I about wore my right shoulder out hand troweling the entire house.
    Your muscles will still be sore. Even rotating the actual sprayer between three people takes a lot of effort, but the speed of application, and the forceful contact of the plaster on damp straw work like a charm.
    I welcome any questions from readers and have photos to show if needed.

  14. Hey Andrew, I have a Question for you, My husband & I have built a 2 story Cobb house & were about to commence useing lime plaster & lime paints as an interior & exterior finish, either trowled or sprayed, some over bare cob walls & some over partially finished earthen plaster walls. Would you recomend this? The above commentary has raised some concerns. Are lime plasters straw bale only?…..

  15. Hi Andrew…. good timing on this. I’ll be FINALLY baling within the next week or so… I think this could be a big time & labour saver! My question is: what about chopped straw in the plaster mix…. does the sprayer still work???

  16. Hi Andrew,
    I am ready to apply the third and final (finish) coat on my strawbale project and, after watching your plastering DVD, reading this blog, and the St. Astier instructions for applying the ecomortar, I am curious as to your opinion of “overworking” the plaster. The instructions specifically refer to overworking with a steel trowel…does that mean one can use a different type of trowel and not overwork the plaster? Seeing as I am a novice at plastering, I probably trowel the plaster with more passes that a pro would. The solution to minimizing the troweling might be to spray it on…have you ever sprayed ecomortar? Any tips? Thanks…

  17. I have not yet tried spraying the Ecomortar. It can be overworked with any trowel. The key is to place it, finish it, and leave it alone. If you over work it, you will separate the water from the material and the fines from the coarse. You want a complete mix of materials to bond well. Overworking can actually wash the lime out of the plaster by bringing it to the surface with the water and then drying it out quickly (surface drying). I think spraying a a quick trowel would work, but I have not done it so cannot say for sure.

  18. Marcy, I missed your message somehow. Sorry. I would be a little hesitant to use the lime plaster over the cob. It can be done, but it is riskier than using a clay plaster. If you do decide to use lime, let me know and I will send you a file that talks about the ins and outs of doing so.

  19. My niece and her husband will be building a 24X24 staw bale barn later this summer and I have eagerly volunteered to help them out. The spraying definitely sounds like a better way to go, albeit not quite as social an activity as packing mud side by side with your friends and neighbors.

  20. I have a six yr old SB home and the plaster is failing on the south and west sides. First we used a mud plaster coat [with lime putty added] then the lime plaster. I over wet cured the first 2 walls and I think that’s what caused the plaster to fail. Now I need to repair the walls. From your info should I just remove all remaining mud and lime and startover with spraying up lime plaster coats? I’m lousy at chemistry and the lime info is hard for me to understand. Thanks

  21. Hi Ruth. I think that might be best. Everything I have read in recent months points towards a mud plaster beneath a lime plaster as a bad combination. If you can remove the plaster all the way down and build it back up, that’s probably the best fix, albeit not the easiest. You may want to get a professional to help you.

  22. I’m kinda new to this and now I’m not really sure what is the recommended way to plaster a strawbale home (interior and exterior)? I’ve read so many different opinions :). What are the reasons for choosing either earth or lime plaster?

  23. Hi Bobes. There are several articles on my blog about the advantages and disadvantages of plaster types. It often comes down to user preference. I hope you find some input in the blog to help you decide.

  24. Good question Charles. I think you’ll nee to make adjustments based on the type of plaster you’re spraying and the thickness at which it is mixed. In general you should mix the plaster as if you were going to hand apply it. You don’t want to add too much water as that will weaken the plaster. You will need a big compressor so that the pressure can stay high without the compressor cycling on and off all the time.

  25. Charles. Heres a great answer to your question that I got from Andy deGruchy, the owner of He’s the guy that I got my sprayer from and he sells tons of Natural Hydraulic Lime on the East Coast.

    Hi Andrew,

    I have found that to have the oomph you need to push the sanded lime material you need at least 24 cfm (cubic feet of air per minute)at 90 p.s.i. continuously. So if you get a 90 psi pancake compressor- sure it puts out that psi pressure but peters out in no time because it does not have the volume of air per minute to keep going and won’t spray sanded stucco without frustration. In order to not have any question regarding having enough volume of air so you can keep working all day long I get the tow behind 85 cfm (if they have them that small) or 125 cfm air compressor (which is more typical) from a rental place.


    Andrew L. deGruchy


    P.O. Box 151

    Milford Square, PA 18935

    Technical help phone 215-536-6706

    e-mail [email protected]

    Material Order only:

    Attn: Jim Plante

    Phone: 215-536-6706

    Fax: 215-536-2281

    [email protected]

    Physical warehouse address for pick-up orders:

    1801 South 12th street

    Allentown, PA 18103

    Attn: Ed

    On Behalf of the St Astier Network the information that is supplied is by way of advice only and does not constitute a specification. Specifications are job specific and generally require a site visit and formal agreement. The information contained in this communication is not for general distribution.

  26. It makes sense that if you follow up plastering with something to even out the coat it would ensure a good seal and a good bond. That way you owuldn’t ahve to worry about redoing the layer for some time. That would save you money and allow you to save some time and money that you can use to do other things.

  27. I thought it was interesting how you mentioned the traditional way to plaster an the spray. It’s really cool that you can do this by hand, since I only thought it was done with spraying. It’s good to know that the art survived with the traditional way, but the effectiveness has as well with the spray.

  28. It’s interesting how you said that spraying plaster gets rid of air while also being compacted. Hiring a licensed plasterer to do this for you would definitely be a good idea. That way you are sure to not ruin your home or something like that by blowing in a wall or weakening the structure.

  29. Hi Todd. Plastering is tough and I agree that hiring out the work is the best idea for many. That said, spraying plaster will not blow your wall down. If it does, then there are major problems in your wall system that simply hiring a professional plastering crew will not fix. Your walls should be tight, strong, and durable long before the plaster is installed. Good luck!

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