Most of our zincs have turned passive having obtained a thick layer of white zinc oxide on them possibly through being in brackish estuary or river water. Looking back they were fairly oxidised a 14 months ago so the bow thruster and prop anodes have been doing most of the work.
Having spent eons of time searching for an alternative to replacing 13 zincs, and having discussed the matter with many in the industry, the great face-off began to recover what are otherwise very good anodes by weight. Having tickled them with a wire brush, a bigger wire brush, a nylon based facing off pad the only thing that made any progress was a small angle grinder with a wire brush which has returned them to a bright zinc surface:
Some new anodes will be making the journey
after fly cutting to fit the hull recess
as seen below, this being one of eight previously calcified hull anodes