Prime before using Topside Paint on:

Bare wood & fiberglass surfaces
Rust-Oleum Marine Coatings Primer for Wood & Fiberglass (for use above the waterline only)
Bare metal surfaces
Rust-Oleum Marine Coatings Primer for Metal (for use above the waterline only)
Previously painted surfaces
Either of the Rust-Oleum Marine Coatings Primers can be used if desired. Priming is not necessary unless paint is cracked, peeling, or otherwise unsound.