Every year I remove the loose laid polyprop carpet and wash it at home and prepare and paint 2 cts of Owatrol CIP (corrosion inhibiting primer) on the ballast plate in my 27,in particular to the edges around the galley/companionway and to small areas in the heads where surface rust has developed.So this holds any serious corrosion and is quick to apply.It is slow to cure however so is best to apply when you are leaving the boat for a day or two.It's purely cosmetic as these plates are so resistant and massive.
Whatever your corrosion inhibiting primer calls for but this year a brass/steel brush attachment in a battery drill did the job in 5 minutes.The intention is to 'hold' the corrosion rather than eliminate it,although that would be nice.
Actually we did see a 26 for sale in Swansea ,when we were looking to buy ,which was particularly unprepared for sale generally and boasted an exposed ballast floor plate with large depressions in it which would have required a new surface screed or false floor,but maybe it had been left awash for years ?
In fact my only concern would be if seawater filled the crevice between hull and the edge of the iron with a long period of corrosion and applied pressure to the hull by exfoliation - but I believe that would only be the case with a mild steel plate.
