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
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.