As far as I'm aware, there isn't a paint which'll adhere to galv.
Brucey's suggestion of an etch primer might be a starting point but, tbh, I wouldn't be holding my breath.
A few years ago I was involved in the refurbishment of the visitors centre for BNFL at Sellafield.
We inherited a staircase with galvanised treads.
Following extensive research, we were unable to find a paint on / spray on product which would improve its appearance and stay adhered.