Remove the loose rust with a wire wheel and use either POR-15 or this Rustoleum product. For all of they hype I have seen over the years about POR-15 in this application, in my experience, it doesn't work any better than the Rustoleum paint-over-rust product I have linked to. Both will wear off a bit in certain areas of the deck.
Here in Pgh Pa. And Johnstown Pa. It’s easier to find these Places. Nanty Glo.
Near Johnstown where i get my Stuff dipped. Very good place to get rusty stuff clean. Last time i was there. I had a 1969 Pontiac Firebird Shell Dipped.
And a 1970 Buick GSX.