The barrier coat is intended o be a continuous membrane, so a patchwork approach probably won't fly unless you're repairing damaged parts of a previous barrier. In any event the barrier should not be applied over old anti fouling, I'm afraid.
If the bottom is blister free and it's just a matter of some spot fairing and sanding, then simply cleaning, painting as jw29 suggested may be all you need for now, and go sailing.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)