I also have been dealing with a rusty tank on my Renault 18i.
I found a tank from another Renault and it was only slightly rusty.
I cleaned it out with gasoline and gravel. The gravel sloshing around with
the gasoline will clean up the scale and rust. You may want to use buckshot,
BB-gun size, but heavier. I also used Muriatic acid to clean more of the
rust off. Be careful with that stuff. After cleaning the tank, some thin
areas and holes emerged. I patched it up with some marine grade epoxy. There
are many fuel tank patching products available. JC Whitney also sells two
types of coating fluids.
In the end you may want to try local radiator repair shops as they can also
fix gas tanks. (not always advertising it)
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