Have you any idea how rusty is the inside of your keel?
Because of the way the keel of the Manta 19 is constructed, the inside of the keel, i.e. the centre plate slot, is normally inaccessible for maintenance but constantly exposed to the water.
As a big winter project, Mekicevica is having maintenance done on her keel.
The first part of the project was just completed: keel removed.
Look at a description of the process and the sad state of the keel on http://mekicevica.blogspot.com/