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In article , says...

Thanks Tony, My keel turned out to have to have 1/8" or more of scaley
rust all over and it is a real chore to get it all off. Thank goodness for
air scalers! I'm hearing several locals make the same suggestions on coal
tar epoxy.

There is only one way to get that scale off and that's with a chipping hammer.
If you grind it it just gets shiny and you think you have bare metal when all
you have is shiny scale. We know from experience. We spent several days just
whanging away with the hammer until all the scale was gone. When that is done
you will probably find that the keel is pitted. At that point you must
sandblast, if possible, or grind down to shiny metal then apply a zinc type
primer. They come in epoxy-type coatings. Then fill and fair the pits. Then
apply epoxy coating. Then apply antifouling. Also not a bad idea to mount a
couple of flat zincs directly to the iron. Then be resigned to having to do at
least the bottom of the keel again next year. Hitting bottom, though, does
keep the barnacles off.

I would never again have an iron keel.