There are 1-part topside paints that are very UV resistant. These are used on boat decks and anything else above the water line. If placed over a red oxide or other rust-inhibiting primer, I'd suspect they'd perform fairly well at UV resistance, even though they're ultimately made for wood or fiberglass substrates.
Used to be Valspar, but they've switched to Majic brand, the last several years. Similar products.
I'm convinced it makes a big difference in weeks 1 - 4, maybe even longer. But it seems to me that the paint reaches a similar final hardness with or without, it just takes much longer without.
If you use Rustoleum be sure to use "professional". bigger can, fast dry. If you can access a paint shaker that's great because it may have been around a while ie. home desperate warehouse
The other thing I do is put the can in a bucket of hot water for awhile. The nozzle is less likely to produce spatter and the paint will flow out more before drying/hardening. I find this is almost a necessity when the paint has been stored in the garage where it is not subject to freezing but is definitely cooler than in the house. I discovered that it takes at least 24 hours to warm up if brought inside the house and left to sit.
That's a good tip! I'd never have thought about preheating a spray paint can in hot water! I'm curious as to how long is "awhile" to you?
I usually do 10 minutes unless it's really cold, then about 20. I use water uncomfortably warm to your hand.