My grandfather used a product called sympaco some 40 years ago on several barns he formerly owned when I was a child. The metal roof's have been untouched since and show no signs of rust at present. I am converting a barn into a house right now in southeastern Indiana and would love to find this product for myself. The paint they often use on trailer home roof's is what many hardware stores in my area have suggested, I tried that route on my barn some 5 years ago and now I have a multitude of surface rust.
If you find a better option regarding paint, please feel free to share it with us here, would appreciate it greatly.
I personally am about to buy all new standing seam material (painted of course) and just be done with it, after all this barn is turn of the century post and beam hand saw action.... hehe