Very early on in our careers as straw-bale builders, we realized that being able to pump plaster was going to be important if we were going to attempt multiple projects. Bodies and spirits just wouldn’t be able to keep up with endless hand-plastering. So, we bought an ancient pump and started spraying.
Oh, how I remember the days of the sprayer nozzle! The comforting “farting” sound, the reassuring overspray sticking everywhere, plaster in our eyes, noses, lungs, hair, shirts and sometimes ending up on the new roof of the house we were plastering (do not trip while spraying!). The nozzle end was a tiny opening 1/2 to 5/8inch(12-16mm), so if a tiny pebble made its way through the screening and into the nozzle, it could (and did sometimes, much to our chagrin) jam up and create back pressure, even to the point of exploding the hose. Luckily no one has ever been in the way of the hose at the time, but what a sorry mess it makes!
Devising a Solution. We talked often about improving the system. I had read about trowel ends for plaster pumps before, and this kind of fitting seemed like it would be cleaner and easier to use, but it seemed impossible to find one for a large stucco pump. As often happens in life, I set about to make my own. The first step was to buy some new supplies:
- 8-foot(2.4m) length of 1-inch(25mm) rubber air hose
- 6-inches(152mm) of 1-inch steel pipe threaded on the outside.
- 1-inch inline swivel (grease twice daily!)
- 1.5-inch(38mm) cam lock coupler, NPT threads
- 2-feet(0.6m) of 3/8-inch(9.5mm) round and square bar
- various 1-inch hose barbs and bushing reducers
- aluminum hawk or similar sheet metal
Then I followed these steps:
- Make a 30-degree (approximate) bend in the 1-inch pipe, leaving an 11inch(280mm) section of the pipe straight at one end. Use an acetylene or propane/ oxygen torch and wind a coupler onto the threads or they will get bent!
- Grind a flat face roughly 3/4inch(19mm) across along the straight, 11-inch section. This is where the trowel will attach.
- Grind a slot through the pipe in the flattened section, 3/8-inch(9.5mm) wide by 5-inches(127mm) long, centred five inches from the bend. The plaster will exit through this slot.
- Place the flat face on the workbench with bend up and weld on four reinforcement bars flush with the face. Use the 3/8-inch square bar. These are necessary to support the trowel attachment, as the trowel material is not strong enough by itself.
- Weld on the handle. Shape to taste from 3/8-inch round bar, remembering that heavily gloved hands will be trying to hold the handle.
- Lay out the trowel face. An aluminum hawk makes decent material. Our trowel has very rounded corners and is 12-inches long by 6-inches wide(305x152mm), with a 3/8-inch by 4-1/2-inch(9.5x115mm) slot. Bias the slot towards the end of the trowel by 1/2 to 1-inch to allow closer application to ceilings.
- Use a drill and saber (jig) saw to cut the trowel out. File off sharp edges.
- The aluminum is fastened to the steel pipe with polyurethane caulking and annealed steel wires twisted tight with pliers. Our earlier experiments using Lexan for the trowel, attached by 20 machine screws failed, lasting only one or two jobs.
- The rest is basic plumbing: use Teflon tape on all threads and heavy-duty hose clamps. As we’re reducing the hose down to 1-inch, a full size quick-connect is necessary at the upstream end of the eight-foot hose to allow for proper clean outs.
- After trying several types of plugs in the open end of the pipe and wasting too much time searching for them at clean out time, we’ve settled into a groove using hand cut plugs made out of styrofoam. They hold just enough that, if the slot plugs up, the pressure pops out the plug. Foam rubber would probably work just as well.
A New and Valuable Tool. Thus was the birth of the power trowel. It worked! No more overspray! We won’t kid you…we still make a mess when we plaster, but at least it’s more controlled now. The power trowel needs two operators (or one if that person is truly a power-power troweler, such as Andrew McKay!). One person handles the hose, the other holds the trowel end up against the wall. The trowel end emits a continuous “ooze” of plaster (hence the nickname “Barfing Snake”), and the speed is controlled by the throttle on the pump.
The trowel can be either moved sideways across the wall, or more popularly, up the wall. If you are using an up-and-down motion, the trowel must be held perpendicular to the ground, catching the material being squirted until you can begin applying at the wall’s base again.
There is quite a knack to this grueling job, and the pairs who are quite talented at it actually seem to dance together as they pass the power trowel back and forth, weaving gracefully around scaffolding, rocks, bales and other typical plastering obstacles.
- fills hollows, good penetration into bales, flattens mud as it applies
- less clogging because of wider opening, can pass fibre mixes
- blow off valve works
- less back pressure, easier on pump engine and workings
- less loss of paste and water to atomization, resulting in longer working times
- no more overspray on windows, ceilings and people (although we do still drop a bunch on the ground/floor)
- overhead areas difficult
- does not quite reach ceiling, trowelers often have to push the mud up the last three or four inches(75100mm)
- occasional air pockets between coats
- somewhat more physical effort for the nozzle person.
We still sometimes reminisce about the old days of the “farting” spray, and will occasionally bring it out of the closet and take it for a test drive; once a friend wanted to record it for a CD, but do we really miss it? Not a chance! The power trowel has made life as plasterers easier, cleaner and quieter.
Peter Mack is a full-time bale builder and a partner in Camel’s Back Construction. He is co-author of the book Straw Bale Building (New Society Publishers). Contact: Peter Mack <email@example.com> www.strawhomes.ca
Articles about methods and equipment for spraying bales with plasters appear in #43 Spraying Earthen Plasters in Colorado), #33 (Stucco Pumping Iron).