I may be wrong here, but my limited observation is that the really bad/deep rust happens on the top side with wet grass percolating in the low spots and corners. The underside gets the recurring surface rust but gets blasted off once in a while. A little scraping periodically helps there.
When I rebuilt my deck, I drilled a few 5/16" holes at the low spots in the deck to help drain any standing water. I blow off the deck with the leaf blower after mowing to reduce the grass agitating the top side paint. I like POR15 (with proper prep) and it will protect the metal on the underside where not directly in the grass blasting zones. But nothing holds up on the high wear spots. I would consider painting the underside every couple of years, anyway. Nothing to lose doing that. Jay
When I rebuilt my deck, I drilled a few 5/16" holes at the low spots in the deck to help drain any standing water. I blow off the deck with the leaf blower after mowing to reduce the grass agitating the top side paint. I like POR15 (with proper prep) and it will protect the metal on the underside where not directly in the grass blasting zones. But nothing holds up on the high wear spots. I would consider painting the underside every couple of years, anyway. Nothing to lose doing that. Jay
