Of all the elements, direct sunlight is the most harmful of all to a paint film...so, your thought of applying a protective coating before it's actually needed is not bad thinking.
Unless paint has worn off to bare, or rusting is an issue, no primer is gonna be necessary. As someone mentioned earlier in this post though, oxidation has occurred (in the form of chalk) and should be dealt with by detergent cleaning. Once cleaned, rinsed and dried thoroughly, I'd personally recommend using a rust-inhibitive acrylic (latex) satin, or gloss finish in whatever color you choose (from your pic it looks like bronze tone) - regardless what the previous finish was. Rust Inhibitive acrylics (sometimes referred to as DTM, or Direct to Metal) have amazing adhesion and will adhere directly to alkyd (oil) finishes without the use of any tie-coat primer. The other advantages to the acrylic finish are oxidation (chalking) will not happen as quickly as with alkyds, acrylics hold their color and gloss far longer than will their alkyd counter-parts, acrylics dry, and cure, faster than alkyds, acrylics won't provide a food source for mildew as alkyds will, soap and water clean-up, more environmentally friendly, etc. etc. etc.
I don't know if Rustoleum still makes an acrylic rust inhibitive enamel, but there are many other brands that do (both regional and national brands) - my suggestion, ask your local independent paint dealer for his or her product recommendation. Good luck, let us know how things work out.
Thx for the input ric. I actually have some black Ben Moore DTM paint in the garage that I thought about. Is it a y more or less effective if the surface is already primed and painted, as in my case?