Alain, the coating is usually PE and will stand the temperature of boiling water. The water tends to pick up some taste. However the outside will rust pretty quickly once it has been exposed to fire. Storing something - especially hard objects - in the tin can will damage the coating. So I would rather spend some money on stainless steel cups.
I wouldnīt store esbit in can thatīs used for food either.
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If it isnīt broken, it doesnīt have enough features yet.