Is the inside contaminated? If it only had beer in it and has remained sealed then it should be very clean. The beer guy would have put a mild bleach solution in it to kill any residual yeast, sloshed it around and dumped it before rinsing with clean water. To ready it for fuel I'd say a soapy solution sloshed around, a good rinse, drain as much as possible then a bottle of HEET to absorb any small amount of water retained when you fill it. Shouldn't need any type of sealer/liner solution for a keg that held beer at pressure. I'd rather have a clean SS tank than one with an internal coating to slough off and cause you problems later.