Just Read, Understand and Follow the directions on the
labels of any/all products you may use.
If the metal is truly galvanized, there are specific primers
for galvanized metal. Then paint with any good quality paint.
If the metal is just raw steel that has been previously primed
and painted, there are specific primers for that as well.
Personally, I like the Rust-Oleum Red Rusty Metal Primer
in the quart can......... Sticks to everything. Dries slow so it
can seep into cracks and crevices to form a good bond in tight places.
Simply remove all loose and flaking paint, wire brush and sand
any bare metal spots.
If you take steel down to bare metal, do not wait 2 or 3 days to
come back and prime it - try to plan your day accordingly - low humidity,
start early in the morning, do not paint anything after 5:00pm your local time
if it is to be left outside. Inside a garage, do not paint after 7:00pm.
Don't overthink it - it is a boat trailer - not your grandmothers dining room table.
again - Read, Understand and Follow the instructions on the label.