ORIGINAL: Charlie P.
Alcohol is hydroscopic and once open 100% soon becomes 95%. It sucks water out of the air. Iso 91% is about the highest stable concentration that is readily available in drugstores. 100% alcohol would have to come from a lab supply store and would cost plenty. Solvent and stove alcohol are about 91% also. Denatured solvent alcohol still has 3% water when sealed at the factory.
Interestingly detailed information there. How soon is soon, and when does it become 90%?
If you'd like to get a feel for how fast it sucks water from the air, weigh a can and open it. After however long you wish, close it and weigh it. There should be a rate of evaporation printed right with the rate of water absorption documentation and you could apply both to see just how fast your can chugged down the water out of the air.
Or you could simply use the least watered down alcohol you can find to start off with.