I've spent the last 5 years doing a
refit on my
steel ketch. Ospho or phosporic acid converts the rust to iron phosphate a black solid that seems to be firmly attached to the
steel, it doesn't remove the rust but changes it. I would try to remove as much rust as possible before letting the acid do it's thing. I am very suspicious about the thin cement, but can't say I have any experience except with the cement ballast in my two steel boats I've worked on. I wouldn't expect it to stop the
water and the rust. Leaving the tank as just
storage, allows future checking of the
hull and rapid access to any problems. I have spent most of my time on the latest
boat gaining rapid access to rather inaccessible places on my
boat, to me that is very high on the list of important things. I replaced most of my
tanks with rigid plastic and made them all removable in a
emergency. Plastic is very easy to weld and heat
mold with a heat gun and such, you might consider making a tank to fit the space out of flat
sheets of plastic cut molded and welded. I put fittings any where I need using a hole saw, a soldering iron, and thin plastic to use as a welding rod. I cut the donor fitting out of a tank and put it where I need it.
The only
tanks that were
water against steel on my boat was the black water and a
keel cooler for the
engine. I am a strong believer in keeping
salt water as far away from any
engine as possible. The black water tank is now plastic and the
keel cooler may still have a problem that will be addressed later. Seems like there may be a tiny leak. When I bought my boat I had to replace many steel
panels in a hurry to get it out of the yard it was in. I think I've got a couple more
panels to replace next time I have it out of the water. You can see a lot of my
work on my website.
I clean all my steel by first beating the heck out of it with a hammer and punches, to pop off any rust, then with a rotary wire brush, repeat the hammer, treat it with phosphoric acid for a day, maybe a second time for another day, then if it's some place that will routinely get wet, I
paint it with a 2 part
epoxy as the first coat, then normally cover with 3-7 coats of
oil based paint. I still have some rust show through in about 3% of the places I've treated like this after a couple years, those place need a redo of the process.
I've reshaped the concrete
bilge areas to eliminate water in the
bilge sloshing against the steel, by making a sump in the middle of the previous flat concrete. I make the sump when I pour the concrete by inserting a 5 gal plastic gas can full of water or something like that into the concrete a couple inches deep. I then paint the hardened inside of the sump with the 2 part
epoxy to seal it. I leave the rest of the top of the concrete unpainted so it can breath. I do everything I can to keep water away from the steel, no matter how it's been covered. I also paint a couple inches of the edge of the concrete and the
steel hull 6 inches up from the concrete bilge with the epoxy. The rest of the
interior just gets the 5 coats of
oil based paint.
I also think coal tar and a
drinking water tank sounds bad. There might be some plastic foaming type stuff that might
work or the epoxy. I've lived with 2 liter pop bottles for water for many years, I still have not had any water in my new tanks yet.
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