The Last Straw Article
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This article originally appeared in issue #39 (Fall 2002) of The Last Straw.

Home-Made Plaster Sprayer Plans

by John Kingsley in Wakefield, Québec, Canada.

In 1999 I was searching for a tool to help with the job of stuccoing our strawbale garage. Thankfully, I found Paul Nabholz's article "A Home-Built Stucco Gun" in issue 13 of TLS. Using his plans I cobbled together a sprayer out of an old metal teapot, bits of wood and plumbing supplies and, in Canadian tradition, yards of duct tape!  While not as elegant as Paul's sprayer, it did the job and I thank Paul for his plans.

During the long winter after the garage was finished I schemed design improvements to overcome some problems I had run into. First, the hopper was difficult to fill, so much so that I had hired my 13-year old nephew to be the official hopper filler! Second, the height of the hopper had made it hard to stucco near the ceiling.  My on/off valve was hard to operate and the sprayer itself was difficult to clean, with lots of spots for mortar to hide. Finally, the sprayer was challenging to build (for beginners).

The major change I made was to design a short wide hopper allowing mortar to be easily scooped up from a mud pan. This article describes this new design.


The sprayer has three major parts: the hopper, the main handle and the top handle. The hopper is made from galvanized sheet metal (same material used for flashing valleys in roofs). The main handle is made from 1½" ABS DWV (drain,waste,vent) pipe with an air compressor blow gun as the trigger mechanism. The top handle uses ½" copper water pipe. All the parts should be available at a most hardware stores for around $40 Cdn.

Building The Sprayer

To build the hopper, first lay out the plans on thin cardboard (bristol board is good).  This allows you to understand how to build the hopper and if needed, make minor adjustments to the shape (e.g. to account for metal thickness). The hopper pattern for this cutout is available at the web site listed at the end of this article.  Once you are happy with the shape, the pattern can be transferred to the sheet metal and then cut out using tin snips. Be sure to file and sand the sharp edges. Then the metal needs to be folded.  This can be done using a vice or two pieces of plywood and some clamps. After folding, check the pieces for fit and make any adjustments (a few whacks with a hammer does wonders!).  Next, drill the hole in the back piece for the nozzle and in the front piece where the mortar will exit the hopper. Initially, start with a small front hole (½") and later enlarge it as needed (more details below). Be sure to align both the front and back holes. Next, assemble the three pieces of the hopper using pop rivets or small bolts.

The main handle is built from ABS pipe, cut to length as specified in the plans. Rather than gluing, join the pieces using sheet metal screws for disassembly later, if needed. To install the blow gun, cut away part of the end of the handle so that the trigger of the blow gun will rest below the main handle and thus make it easy to operate. If needed, modify the blow gun with a grinder or hacksaw to allow it to fit into the ABS pipe. Drill a hole into the end cap of the main handle to attach the blow gun to the end cap using a quick-connect air hose attachment.

Use a flexible plastic hose to connect the blow gun to the nozzle assembly. The type of blow gun you purchase affects how the hose will attach to the blow gun: you may be able to find a barbed connector to do the job, assuming the blow gun has standard threading. If not, perhaps a smaller barbed connector can be soldered in place or a larger connector can be drilled or ground to fit and then also soldered in place.

To attach the other end of the hose to the nozzle assembly in the hopper, cut an access hole in the top of the ABS pipe at the end of the main handle to allow a wrench to be inserted to tighten the connection. This access hole can be covered with some sheet metal and some screws to keep the mortar out of the handle. Finally, the main handle attaches to the hopper using some screws.

The top handle is made from ½" water pipe. After soldering the pieces together, attach it to the hopper using bolts, and to the main handle using screws. Be sure to "dry fit" the pieces before soldering to make sure everything fits together snugly. Crimp the end of the top handle, where it attaches to the main handle, to make a good connection with the ABS pipe.

Once the sprayer has been assembled, you should make some final adjustments.  First, the front hole should be enlarged.  In the plans I say it should be 5/8"; however recently I have found 3/4" to be better. Do some tests with mortar and adjust the hole size as needed. A small cone-shaped grinding stone placed in a hand drill is helpful when making these large holes. Also, the front sloped side of the sprayer should be "adjusted" with a hammer to allow it to cleanly scrape mortar off the bottom of a mortar pan. The idea is that you should be able to scrap the mortar out of the pan just like a flat shovel.


I operate my sprayer using a 5HP air compressor. This runs on regular electrical power, i.e. 120 VAC.  I'm fairly sure a smaller compressor can be used with the sprayer, but I haven't tried this.  I use about 80 PSI although this will depend on the air hose length and mortar thickness. When operating the stucco gun, be sure to wear a filter to protect your lungs, eye protection, hearing protection and also rubber gloves.

The sprayer is ideal for applying the first coat of plaster where you want a thick layer and also want to have the stucco bond well to the straw. Compared to applying stucco by hand using a hawk and trowel, handling the sprayer requires little skill. Also, it demands less effort from your body - your shoulders and wrists will thank you.  However it does have its drawbacks: it is messy  - you must protect your windows (and your body!). You also need a compressor to operate this tool (however these can often be rented). Finally, it's noisy.

The sprayer has been used to apply portland cement-based mortar to a strawbale wall and also to coat a surface-bonded block wall.  The sprayer handles fiber with no problem - I have tried both polypropylene and fiberglass. I have done some experiments with earthen plaster and had some cracking, but I'm not sure if this a problem or not (due to my inexperience with earthen plaster). Likely some more sand and straw added to the earthen mix would minimize the cracking - more experimentation is needed by more experienced folks.

In this age where it seems that everything is being patented, I'd like to share this tool for free just as Paul Nabholz did in issue 13. I think this is the real spirit of strawbale building. And I hope this article sparks some new ideas which people will share in future issues of TLS!

Full plans and pictures can be found on the Internet at

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