I used the OSPHO on my C22 keel after grinding it down to bare metal. Next, I used WS Epoxy. (BTW WS Epoxy IS barrier coat. It's one of several barrier coats out there). If you use West, just be sure to scrub it with soap and water and a scotch brite pad before you go to the next step.
For me, the next step was to build up a better foil shape of the keel using a fairing compound. After fairing the keel, I put on several more coats of epoxy barrier coat before painting with antifouling.
I did all that in 2007. Since then, the boat has been in a slip except for trailering to other sailing grounds, and hauling out for repainting the bottom. So far, no rust.