This page on spraying stucco with compressed air started with a question originally submitted by Dennis, from Chatsworth California.
"Regarding spraying stucco with compressed air, I was looking at stucco hoppers to attach to an air compressor to do some re-stuccoing on a house.
The specifications stated on a particular hopper/spray gun kit, stated it's air flow/pressure requirements at seven (7) CFM at twenty five (25) PSI.
Since most compressors measure their flow based upon either 40 or 90 PSI, how would I compute CFM at 25 PSI?
The sales personnel at Home Depot were unable to help me.
Regarding your question about spraying stucco, if your compressor air tank is big enough to have enough reserve to keep supplying the stucco gun while the compressor is working to keep up with the compressed air demand, you'll do fine.
Worst case is that you'll have to stop spraying for a while periodically as the air tank gets depleted and the compressor tries to fill it back up to the compressor cut out pressure.
I know that if you're spraying stucco for pay, then time wasted while waiting for the air compressor to recharge is money lost. That being the case, you need to get a bigger compressor and tank.
I'm going to have to say that I've never tried to compute the variance in flow of a compressor. I know 1HP equals about 4 CFM @ 90 PSI.
I actually used a small 3HP compressor this morning to stucco spray the ceiling in a small bedroom at my home.
While the compressor was able to supply the air pressure required of the stucco gun, where it fell down was on the volume to the gun through a too small air hose.
If you are using a small compressor to supply a stucco gun, you need to make sure the air volume to the gun is according to the gun's spec. Get and use a larger I.D. hose.
My cheapee home compressor came with a 3/8" hose. The stucco gun spec called for a minimum 1/2" hose. As a result, I had problems with the viscosity of the stucco plugging up the nozzle orifice from time to time, and the stucco not spraying as smoothly as I would have liked.
I could still use my same home compressor and buy a 1/2" RVC hose to use with it (this would mean that the compressor would cycle more often than it did with the 3/8" hose) but the 1/2" RVC hose costs about 1/5 the price of what the compressor did, and I'm too cheap to spend that money at home.
Course...if the company was buying...?
Good luck with your stucco job.
P.S. A bit of an update...I broke down and bought a 1/2" RV (rubber/vinyl) hose to get more air to my tools. I had to buy some step down adapters, but now I've got enough air flow to the tool (stucco sprayer among other things).