Doing the base plate is a grim job, its worth getting a cheap crawler board to lie on. Did mine last year with an angle grinder and a large heap of coarse flap disc wheels thingys. It was reasonably quick and not too dear and did an acceptable job. Tercoo discs work very well but they are slow, I have never tried twist knotted wire brushes. I used Jotun 90. The manufacturers reccomended no undercoat or primer and to brush it on (Not roller) I did the sides with Tercoo discs in a drill and they are good but as I said, slow. I was not really removing old rust though, it was mostly barnacles and the fibrous parts of mussels, Bee was in brackish water for a couple of years and these things are really, really hard to get off and there were thousands of the little sods. Good luck.