The electrical contact area is really only the bolts, by design. This works as long as the anode doesn't wear away around the bolts, which if installed without a care it will do pretty quick.
Removing protective paint from the saildrive itself is counterproductive, as that would increase anode wear (larger exposed surface to protect) and not solve the problem of the zinc eroding around the bolts.
What I do is apply waterproof grease (Volvo propshaft grease in our case) on the anode where the bolt heads sit in the bolt holes and the bolts. This does not prevent electrical contact, but it does prevent seawater getting at the zinc in this area. Therefore the zinc there won't erode and the anode will not rattle lose and stop working.
The rest of the anode does the job and we're in the third year of doing the above now, with good results (two years in the water between anode replacement).
