Suggestions on proper coating to use on Outdoor equipment steel cabinet
corrosion. Vertical surfaces only. I replaced the top with
polycarbonate material.

I need to stop corrosion and protect the surface from further corrosion.

It currently has a powder coat finish that has gone bad after 20 years.

It is all about preparation:
a) Disassemble, to the extent possible. Screws, nuts, bolts, and so forth.
b) Remove all existing coatings. With chemicals, if necessary.
c) Remove all existing rust. By chemical or physical means.
d) Rent or borrow an electrostatic painting device, purchase the appropriate paint. This is NOT a powder-coat.
e) Paint per the directions. Include the fasteners if not already plated, or replace the fasteners with stainless steel.
f) Obtain magnesium washers ILO those supplied, and use them during the reassembly.

Should give you at least another 20 years.

Peter Wieck
Melrose Park, PA