Lime Plaster and Clay Slip Pump
I hesitate to say much about our method since it's a bit finicky to use and for some people and some buildings it would be more trouble than its worth. However, even carrying one half-full bucket of mortar to the highest level of our staging would convince me to use our system.
The system is a pressurized tank, a hose and a nozzle with a trigger controlled air supply to forcefully expel the mortar through a 1/2" pipe.
We repaired the pinhole leak in the hot water heater tank which was found along a road in the trash. We cut the opening in the end as in the pictures and smoothed the inner surface so it would seal with the wooden lid. One has to put the pieces of the lid inside and then fasten the 2 halves together, since no rigid lid can be slipped in through the hole. It is made of four glued and screwed together pieces of 3/4" wood. A piece of inner tube is the rubber seal seen in the photograph. Note the wire in the photograph which keeps the lid from falling in the tank. One must keep the seal area reasonably free of sand. Sometimes it won't seal and you have to flush the lid with water to clean it. But that adds water to the mortar which you don't want. We use two 2x4s across the top of the tank and tighten down the wing nuts like a pressure cooker to seal the tank. This type of lid seems safe since it cannot fly off to hurt someone. As soon as the pressure begins to rise the tightness of the seal improves.
We mix the lime plaster or clay slip in a concrete mixer and dump it directly into the tank (from our bridge). A piece of flashing can be used as a shoot. This can keep the mortar from getting on the lid as it flows in. The consistency of the mix is very important and often we missed getting that right. Too thick and it will not come out of the hose; too thin and you have drippy stuff on the wall. It should be thin enough that some mortar will fall off your glove when your hand is lifted out of the mix.
The tank has a fitting with a pressure gage used on construction air compressors. The gage lets you know if you are getting a seal when you pressurize and it lets you set the pressure you want (~40 lbs.). The compressor hose was fastened via a quick connect fitting at the pressure gage. We connect it when we are ready to pressurize the tank. The outflow is a hose from the bottom of the tank. We found that a slightly larger than usual diameter hose worked better. It may have been called "contractor" grade of hose from the hardware store. We used a 50 ft. hose and went up about 16 ft. above the tank.
The sprayer end consists of pipe fittings: a pipe to hose thread fitting that was free turning (making things much easier to manipulate) going into a gate valve; a 3/4' tee with the gate valve coming into the middle of the tee; a compressor line in on one side the tee -- outlet pipe on the opposite side. The outlet side has a reducing fitting going to a threaded 1/2' pipe 3-4" long. The compressor line has a trigger control to give air as needed. Ball valves which can be closed quickly jammed and broke from the sand, so we used the gate valve as noted above and a 1/2' cap on the end of the outlet pipe. Those two controls let you hold back the mortar with the gate valve or blow out the line into the tank when the gate valve is open, the cap is placed and the compressed air is applied.
Always blow the line clean when you stop spraying. Also, try to avoid stopping the flow of mortar during a run because getting started again often requires blowing out the line. The line can be blown out even with the tank pressurized if you set the tank pressure at 40lbs and the compressor at 100. That lets the pressure you have at the nozzle in the second air line be strong enough to blow out the line. If it won't clear you can depressurize the tank (by opening its valve - not the lid) and blow things out. If blocking up or too slow delivery of mortar is a problem try making the mortar (or clay slip) thinner.
Application: We blew on the clay slip and most of the lime plaster before learning that you can also lay down a bead of plaster like toothpaste without any air blowing. The slip surely needs blowing since that gets it into the straw nicely. Also the overhead areas such as inside windows needs blowing to make the mortar stick. We gave a very generous coating of clay.
When the slip or plaster starts to come out of the end of the pipe (it comes pretty slowly unless the mix is too thin) start to give air from the compressor control. Sometimes full blast is needed, sometimes less is best. The angle you hold the outlet pipe influences how much it spits or "burps". Keep a bucket nearby to catch the flowing mortar if you have to stop for a moment to move the hose or something. That avoids stopping the flow and the potential difficulty of restarting. As the tank gets nearly empty the flow will speed up and at the end becomes a torrent. We close the gate valve before the torrent starts because that mostly blows mortar all over the place. Blow back the line after depressurizing the tank so things will start right with the next batch.
The "toothpaste" method is quiet (quite a relief); it requires accurate control of the nozzle, however. It is often possible to lightly drag the tip of the outlet pipe across the wall leaving the line of "toothpaste" on the wall.
Final points: Be very careful not to let stones into the tank. Some of our clay had stones in it and that caused no end of headaches with clogged pipes. After that experience we even put our lime putty and sand through a screen. We use a hoe to agitate the mortar in the bottom of the tank a little with each load because the sand tends to settle and block the outlet. Since lime plaster does not harden as long as it is wet, the tank can be left for days with its lid in place. Just be sure to use the hoe to agitate the mortar in the tank before you add the next batch because it will have settled around the outlet.
2003 Observations
On the inside walls we are spraying the clay and almost entirely using the "toothpaste" method for the first and second coats of lime plaster. For the last coat of lime plaster, which is the thinnest and requires the most careful finishing, we seem to prefer to put it up by hand.
We have been having very good luck with the lid - it seals nicely almost every time with no problems. This may be due to our mostly keeping the mortar off of the rubber gasket and leaving the lid sealed tight between uses so the lid keeps its proper shape. Also we now always seal the lid in the same position so any shape differences are not a problem.
Hose clogging has presented some problems but but key issues to reduce it seem to be: