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1) pv = nRT 5 cfm of air at 100 PSI is the same mass of air per minute as 10 cfm at 50 PSI The energy required is proportional to the total mass of air delivered, so the fixed sized motor on the compressor should be able to deliver more of the "low energy" low pressure air than the "high energy" high pressure air. It doesn't follow the ideal gas law perfectly due to its fixed size and compression ratio, but will deliver more air at low pressure. 2) Assuming you are using a pressure pot, a 3/32 nozzle takes 7 cfm and cleans 0.5 square feet per minute. a 1/8 nozzle takes 15 cfm and cleans 1.0 square feet per minute, and a 5/32 nozzle takes 25 cfm and cleans 1.5 square feet per minute. For stripping paint from something as big as a tractor, a 1/8 tip goes about as slowly as I would want to bother with. The tips wear as you use them, so the CFM needs go up over time (as does the cleaning rate). If buying a compressor with a lot of sandblasting in mind, don't bother with anything less than 15 cfm. As a point of reference, my 5 horsepower 2 stage Ingersol Rand compressor runs flat out driving a worn 1/8 tip. It isn't really quite enough compressor for this workload. If I were buying another compressor for sandblasting, I'd go up to at least 7.5 horsepower. 3) Two stage compressors have two cylinders. The output of the first cylinder (now higher pressure partially compressed air) is fed into the input of the second cylinder. Typically the interconnect pipe between the cylinders is finned to create an aftercooler. The two stage approach is more efficient, and also enables the production of output pressures higher than any single stage system can reach. The receiver tank stores the high pressure air, and thus represents more stored energy at 175 PSI than at 125 PSI. This gives you the potential for a "burst of power", obtaining more CFM (for a while) than the compressor can produce steady state. Two stage vs. single stage is pretty much moot for sandblasting at 90PSI. What you need is 15 to 20 cfm at 90 psi full time, however the compressor gets there. A $900 compressor will get you in at the low end of suitability, and $1200 to $1500 would be fine for hobby level work. I recommend looking around for reasonably priced special purpose blast media, and not using beach sand. Regular sand contains free silica, which can cause silicosis. This is the reason why OSHA requires supplied air respirators for sandblasting. Around here, we can buy silica-free crushed recycled glass for $0.10 per pound. Other locales have such products as "Black Beauty" (crushed coal slag) with similar benefits. 700 pounds or so will do an N-series tractor without attempting to re-use the media.
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