As mentioned previous for cast iron keel and to do the job properly, I feel you have to get back to bare material.
Grinding will only make the situation worse as it will simply bury the antifoul further into the substrate. Wet sanding is okay but you will still have problems in the short term, as you are still going over leftover product which does not offer a good 'key' for the new paint system. Again paint strippers will only remove so much of the old material.
The first pic is the Sodablast rig which is a wet set-up that will run through a waterblaster. This requires a minimum flow rate of about 20l per min of water to be effective. It is simply graded Bicarb soda and with any wet blasting you must not let water backflow into the hopper or bucket.(goes hard/lumpy)
This stuff is good for the general underwater hull areas and will not ruin the gelcoat which is important. You do not want to be ripping into any fibreglass layers!
The second pic is the waterblaster setup with the grit nozzle which again simply sucks the media out of a bucket or hopper by venturi effect.
You do not have to spend big money these days like this set-up below. Mine is all Aussie Spitwater gear.
What with the cut price imported stuff and a trip to say a Gasweld or whatever, you can be off and running quite cheaply.
Kennards may even hire this equipment out, depending on your location.
To stop water running back into your product, the lance must be held with the tip facing downwards and have the inlet elbow on top(not like the picture).
This used with say a Garnet should clear off the iron keel of any coatings quite well and ready for a new paint system. Get hold of a piece of scrap metal plate as a test piece and try out before howing into your keel to gain a 'feel' for what's required. Try also to be accurate especially near the hull/keel join and some minor filleting may be required as fairing to make good again. Don't use the Garnet on the fibreglass hull!
Lastly here is a sample of the Garnet that I have used on iron keels. The benefit of wet blasting, is most of the waste can be reasonably contained.
With dry blasting, a full enclosure may be warranted,otherwise crap will go everywhere, including all through your's/neighbour's boat. This set-up usually requires big compressors and large equipment also.
By all means consult with a marine paint supplier and get hold of a reputable paint system, including your barrier coats, primers and so on.
Craig, speak also to Windcraft on your Hanse, as you would hope they should be able to offer decent advice on this as well.
If still not overly confident,there would be specialist blasters(probably Soda) around, that do this type of marine work and do your research on their reputation.
Be there at all times to supervise and pull them up if you are not happy. You do not want a Cowboy or an untrained bunny indiscriminately ripping into your pride and joy! C.R.
.
