Need help with choosing the toughest Rustoleum Product

One of the parts of what is the standard bee hive in the United States is the cover (in the UK, they actually use something that looks like a peaked roof with shingles), which has a piece of galvanized sheet metal that is nailed onto the top. These take a horrible beating and I've been repainting the hive boxes this winter and the covers are just in bad shape, some with rust and some just looking bad. Now, on the hive boxes and the bottom boards (where the bees 'take off' and 'land', the standard is some sort of light colored latex paint. But I want to do some rehab on those covers because sheet metal is expensive. I don't know if anyone else has had any experience with galvanized sheet metal that has to deal with sun, rain, freezing cold and horrifically hot temperatures, ice and snow, but I'd like some help choosing the best Rustoleum coating. Yes, I know I will have to take steel wool to the covers and clean them and get rid of any obvious rust before I do this, but does anyone know if I'll need something else, another treatment before I put on the Rustoleum product? I realize that this is definitely not a sexy project, but it certainly will be useful to me because what I'll do when we buy new hive bodies and covers is that I'll paint them with the product right from the beginning.