My immediate thought was Rust-Oleum clear spray, but rustoleum, krylon, and any other alkyd-based paint will react with the zinc and flake off. Primers cover the patina and are therefore not useful. I have not been able to find any other clear paint that would work for this.
Context: This is a decorative application so I'm not sure if this is the right venue. I've added a black patina to galvanized steel pipe using a copper sulfate solution. I need to add a coating to protect the patina and prevent it from yellowing over time.