There are a few signs that you should immediately discard your tank: if it has rust on the bottom, if the valve is stuck shut and difficult to open without tools, or if it has ever accidentally been overfilled. You can exchange your malfunctioning propane tank for a new one at most grocery or hardware stores. A good way to prevent your tank from rusting is the keep it out of direct sunlight while in use — if the tank is warmed while it’s cooling, it’ll sweat down the sides and potentially promote rust at the bottom.