Dave, that looks like it is galvalume? if so, you will need a special paint since galvalume is made to repel most anything, if you want it to last for a while. (even looks like someone painted it and it is peeling)
We get an special enamel for jobs just for that kind of metal from Sherwin Williams. might go to one and see what they can do to set you up. You will be much further ahead doing it right the 1st time. prepping it once to paint is bad enough.