My standing seam steel roof is badly in need of repainting. The paint is chipping in some places and there is some surface rust in other places.How much prep before repainting? Do I need to clean to bare metal where there is rust or just remove the loose rust and paint? Also,the paint store tried to sell me regular metal paint in the color I wanted (green to match the original). will that work ok or is there something better suited for roof paint? Thanks for any help.