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.
".. 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)
1984 Fast/Nicholson 345 "FastForward"