Things to remember when repurposing any container with small standard openings
• Always assume that a container may have held a flammable liquid or gas in the past.
• Always assume that a residue may still be present and can produce an explosive vapour.
• Always fill the container to the absolute brim with water to eliminate any air pockets that might harbour an explosive vapour before you cut, weld, drill, or otherwise apply heat to the container.
• Do all your work on the container with it full of water.
• If you are opening the container, ensure that the opening is of a sufficient size to prevent any gases from compressing within the container, before removing the water.
These precautions should apply to every container, irrespective of size and construction, including all pressure vessels.