I would definitely leave the phosphoric in, as that will convert any microscopic rust to iron oxide, and then clean it with a degreaser to get anything off that will keep the primer from sticking, and then immediately shoot it with epoxy.
As Stuart said you will then have a finish that will outlast you unless you go to the beach a lot.
Using the phosphoric is no big deal. Lay plastic drop cloth on your concrete floor, put old clothes on, put rubber or latex gloves on and goggles, and spray and wipe. I do it on just about everything metal I make or refinish. It kills the rust. It's good stuff.
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