I found a tar like substance on the inside of a 20 lb propane tank I used for a furnace casing. It was not soluble in water or detergent after many cleanings. This substance had released small amounts of propane odor for several years after the valve had been left open, and before the tank was cut. I don't know how you feel about that, but just thought I'd mention it.
(Also, FYI my tank was cut open with a grinder -- not a torch -- while filled with water and the valve removed. I realize this is not about cutting a furnace casing, but using an intact propane tank as an air receptacle, but thought I should include this detail, in case someone tries cutting one for a furnace on reading that I had done it. Very important when cutting a tank that it be filled with water, and a large opening for the remaining small volume is available.
Many people on other fora have confused rinsing a tank with water before cutting, with filling with water while cutting. Totally different situations. And if cutting an unfilled tank with a torch with an organic tar inside, is a recipe for vaporization of the organic, and explosion.)