How to remove the propane smell out of a tank?

I have an old propane tank that I have emptied completley removed the valve, and am in the process of converting it into an air caddy, (portable air tank). There is nothing at all left in this tank, I have filled and emptied it with water a half dozen times but the smell of propane is still in there, any clue as to what to use to remove the smell I dont want to use it if the air coming out of it smell like propane im scared if it smells like propane it could still be flammable on some level, I am still treating it as if it is flammable fumes cause im sure if I still smell it there is got to be some left although unlikely cause of all the water ive been putting in and out.