I make ice shack stoves from old propane tanks. The residue definatly will ignight if the tank is repurposed improperly. The method I use is to knock the valve off, then fill the tank with water, I've made an copper pipe elbow for my garden hose, while filling with water I flare off the gas as it is displaced. When the tank is full I empty it, then fill it again with water and dish soap to remove the stench additives and the last of the residue. When I empty it the last time its all full of suds and cuts fine with either an angle grinder or plasma.
Could you show us some pictures of your "Shack Stoves" ?
