To start with and this is very important, a 100 pound propane tank that is completely drained of propane and should be aired out for at least several months as you will be welding to this tank.
If it’s rated for the pressure you will exert with a compressed air system, then I can’t see why not, as the tank doesn’t care if it’s liquid propane or compressed air inside.The compressor portion is straight out of a refrigerator. Besides the compressor being free, the other benefit is that it is super quiet!
Removing the old propane valve is the most difficult part of the conversion. After removing the valve por in the rust preventative paint put the valve back on temporarily and roll the tank and turn it end over end several times so the whole insides of the tank gets coated with paint.
Remove valve, drain out excess paint and let the tank dry for several days before adding new pipes and valves. Include a pressure relief valve.On the output side of the tank resides a pressure switch for automatically filling the tank and a regulator for ensuring the solder paste applicator gets the required pressure.