Underside of my fuel tank

[Groover]

So I've had the fuel tank cleaned and coated for our 64 Convertible and we've cleaned off the exterior as well.

I was thinking of painting the whole thing with some heavy duty rust-oleum, but I'm wondering if there is some recommended rubberized undercoating for the bottom of the fuel tank and if it can be applied after painting or should it be applied directly on the bare metal.

??

[Ratical]

I prefer to put it over a painted surface.