I’d take a picture of the tub problem and show it to the nearest paint shop.
Chances are that if they don’t have what you need, they will direct you to the store that does.
I’m sorry I don’t have the information that will help you. We have used a tub enamel in the States. It came in a very small bottle - a bit larger than nail polish bottle size. And it had a small brush attached to the cap, also similar to nail polish bottles. Another possibility, for a larger area, is to use a metal paint. The brand name paint Rust-oleum would be good. I’ve used it on metal outdoor chairs.