From my research, there's not much out there that claims to work right out of the box (bucket?) on galvanized. Even paints specifically marketed as being made for galvanized generally require special surface prep, which usually includes etching with phosphoric acid (ospho), followed by an aggressive clean-water rinse to rid the surface of any remaining acid. Many people don't read and/or follow the TDS (technical data sheet). If a given paint is compatible with galvanized, it will be stated there, as well as the specific procedure for prep. I'd follow such instructions to the letter.
We actually just received most of the paint for our exterior. We're going with Axalta Corlar 2.1ST epoxy primer, over which will go Imron 3.5hg+ polyurethane. The primer is direct-to-metal, as well as being compatible with many cured coatings, so I expect it to work great for the mix of the two I'll be coating. It's also a mastic (high build, sealing) primer - suitable for waterproof sealing (immersion) given sufficient film thickness - so it should work great for sealing up any hair-width invisible cracks that may be present around rivets & seams. Per the TDS it's good on galvanized (so long as prep is done). The cost is lower than most automotive-specific 2K products with similar properties.
Another big plus on both the primer & paint is that - unlike many other industrial / automotive 2-part products - the TDS includes specific instructions, and Axalta makes special reducers & additives, for roller/brush application. Most of these types of paints don't, so this was pretty big to us. Not only is spraying impossible where we're at (overspray concerns), this **** is nasty (health-wise). Rolling makes it way safer (though we'll still be wearing full PPE applying it). How it will turn out I can't say, but we're more concerned w/ durability than appearance, so if it isn't perfect, no sweat. We're going to try something akin to the 'roll & tip' method popular among boat builders to hopefully get a somewhat decent look. Another huge plus regarding the Imron is that you can recoat over cured paint without any surface prep other than cleaning. Not many such paints can or do make that claim. Should be a benefit for keeping it looking & protective well in the years to come with minimal hassle.
A great product (per the specs) for sealing seams, painted or bare, that you never want to come apart is 3M FC550 adhesive/sealant. But I stress the never come apart bit. We'll be using 3M automotive seam sealer (or dynatron 550... maybe the same thing?) for stuff that may need to come apart sometime in the future. All are polyurethane based, & paintable.
I'll be painting the roof here w/in the next 2 weeks unless something goes very funky. I'll post my impressions / observations in our build thread (see our sig line) when that happens.