OSPHO is a rust convertor, POR15 and RustBullet are encapsulators
OSPHO converts rust into a different compound - a dark grey harder substance that holds paint well. POR15 and alike encapsulate rust and present a surface to paint. Neither would seem applicable to rigging stainless.
OSPHO does not seem to be much of a penetrant and when it does hit rust it makes it into something harder, which if you are dealing with threads or moving parts would be bad.
I'd use some sort of penetrating oil to soak rusty parts -- LiquidWrench, PDBlaster, or my favorite Kroil.
Bob Pingel
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