Alistair, I found a website with information on restoring old Jerry Cans.
http://www.90thidpg.us/Equipment/Projects/WaterCanResto/index.html

The article tells that the lining of the water cans was food-safe.�
However, sometime in it's past it could have been used for leaded gas, so relining the can is recommended for either food (potable water) usage as hazardous chemicals or metals .
Another thing to consider is that if it's always been used for water, and the lining will hold up to gas, the can may �have a future owner that uses it for water not knowing it was used for fuel before he purchased it.
The type of cap used on the water can is not leakproof.
I see these selling on Ebay for 20-30.00 or more. Maybe just sell it and buy a gas Jerry Can?
Robert
1982 Westfalia