Hello Mike, when you say "retired" are you referring to a tank that no longer passes inspection for refilling? If so you have to understand that these tanks are designated as such for a reason, they may have excessive amounts of internal corrosion, possibly cracks, etc. Granted, you may only be talking about pressurizing this with 100 to 125 psi. instead of the 2250 to 2500 psi. that they are normally designed for, yet, this can still pose an explosive hazzard even at these lower psi's. So if this is the case I would highly recommend avoiding this scenario! On the other hand if this is a good usable cylinder I would still not do it, instead, I would sell it for the going market rate and buy one of those portable airtanks that are really inexpensive and designed for this purpose already. If you would like other good reasons for going this route take a trip to your local welding gas supplier and inquire with them for other reasons. My $.02 Regards, aevald