I have heard good things about POR, 'though I have not used it myself.
I have been maintaining a steel hull for thirty years, and I just use any surface tolerant two part epoxy formulation. All the big names make one - currently using BarRust 235 by Amerron/Devoe, now just called Devoe 235.
Best practice is to sand blast the area, then apply 3 or 4 coats of epoxy, followed by antifouling of your choice. In cases where sandblasting is impractical, I just hit the area with a coarse grinding wheel and apply the epoxy.