There are thousands of 40+ year old mild steel tanks. As mentioned before keep the exterior of the tank dry and rust prevention maintenance. Connecting the tank to the zink loop helps protect the tank.
Also reduce the moisture water in the tank by using turning the fuel, keeping the tank warm, and additives to prevent separate and absorb moisture water into the fuel. Fuel polishing helps as it clean and mix the fuel up. Lastly changing the trim of the boat so stuff does not settle in the same place.
So to have a tank last it has to be maintained.
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