Have you a product for galvanized metal box gutter that has pin holes but is still intact. I was thinking of a liquid resin with fiberglass that could be brushed on as gutter is not easy accessible at back.
Assuming the galvanised gutter has any corrosion removed, it is clean, dry, sound and prepared as detailed in the technical data sheet we can recommend Rust-Oleum Mathys Fillcoat Fibres.
The gutter should be thoroughly degreased and primed with Rust-Oleum Mathys Fillcoat prior to application.