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Old 03-06-2015, 08:59   #1
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Cementing the watertank on a steel boat

Hi there,

So I've finally gotten to the point in the refit of my old steel classic where I'm starting to fix up some of the creature comforts. Next on the list is fixing the water-tank so I can start using the foot-pump and faucet again instead of relying on plastic bottles.

So I refitted the pump, replaced all lines/valves and tackled the 40-something bronze bolts holding the cover to the watertank (who does that?! 40 bolts for a small cover in a metal softer than any screwdriver?!)

Anyway, long story short, I found a few pounds of rust-coloured gunk on the bottom of the murky body of water found under the cover. The inside of the tank seems to be covered in a black, bitumen-like paint. That paint is beyond repair (tiny, rice-sized rust-bubbles cover the whole surface), so I'm on the market for another solution to another unsuspected problem.

The walls of the tank are also the hull of the boat. Due to the shape of the tank, it's completely unthinkable to clean it up again to bare metal in order to treat it properly (yes, even if I cut out the whole top with an angle-grinder). The way I see it there's two main options:

1. Cut out the top, forget about the watertank and use it as storage.
2. Use the tried and true "cementing" of the existing rust/bitumen surface

This last option sounds good, almost too good to be true... It's supposed to counteract any rust, it's supposed to stick to pretty much any surface, it's supposed to be super-easy to apply and it's supposed to make your water taste super-good.

My main concern is that it won't stick properly to what's under it, or worse yet, that it will crack due to vibrations or the boat hitting something. In that case the water won't only be spoiled, it will also cause rust issues on the hull...

Can anybody comment on this? Is the stuff really as good as they say it is? Do I have nothing to worry about?

Thanks!
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Old 03-06-2015, 11:02   #2
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Re: Cementing the watertank on a steel boat

in your case I would pressure wash the tanks,then etch the rust away with acid,then pressure wash again,dry then recoat with bitumen,cement will not stick to bitumen,is very heavy in the thickness you would need.
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Old 03-06-2015, 14:52   #3
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Re: Cementing the watertank on a steel boat

So just cementing it over in the usual fashion would not work in your opinion?

Your option is certainly a valid one, and surely less work than trying to get all the bitumen blasted off, but I'm still not sure it's worth it at this point. Filling the storage with waterbottles wouldn't net me tht great a loss, and I honestly wouldn't mind (also saves water probably and gives you a better idea of what you're using). Added bonus being the pump being re-routed to pump seawater in the galley.
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Old 03-06-2015, 20:32   #4
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Re: Cementing the watertank on a steel boat

When I built our boat, the fuel tank in the keel was rusty by the end of construction.
I filled it with a dilute phosphoric acid solution and left it for a week or two until it was clean steel.
Cannot remember the strength but if you have the patience you will end up with clean metal.

Regards,
Richard.
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Old 03-06-2015, 22:34   #5
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Re: Cementing the watertank on a steel boat

Maybe clean it up as best you can then fit a bladder into the original tank
Chris
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Old 03-06-2015, 23:03   #6
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Re: Cementing the watertank on a steel boat

Since you are talking about a 29 foot steel boat, you are discussing what must be rather thin steel to begin with, or the boat would be too heavy. You are also mentioning rust bubbles in the existing paint job that may or may not be serious . My suggestion would be to cut the side (yes the hull) of the tank out and sandblast the heck out of it , to see if the metal is comprimised. If the metal is all still good, paint with a modern paint and reweld the side plating that you removed, and you will only have the weld lines to deal with in repainting. If you cut the top of the tank and sandblast, you will have sand everywhere in your boat. Rusty steel is a PITA, but steel is fairly easy to repair. Best of luck. ______Grant.
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Old 04-06-2015, 01:24   #7
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Re: Cementing the watertank on a steel boat

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Originally Posted by Orchidius View Post
So just cementing it over in the usual fashion would not work in your opinion?

Your option is certainly a valid one, and surely less work than trying to get all the bitumen blasted off, but I'm still not sure it's worth it at this point. Filling the storage with waterbottles wouldn't net me tht great a loss, and I honestly wouldn't mind (also saves water probably and gives you a better idea of what you're using). Added bonus being the pump being re-routed to pump seawater in the galley.
i would think a thin layer of cement would quickly crack from the on going corrosion beneath,leaving you with rust coloured water again.

to stop corrosion you need to stop the air getting to the steel,after it has had the rust removed,cement is quite porous,and would possibly make the problem worse.

as jorden suggested the best remedy might be to cut out and replace the bottoms,however the method i suggested would be the easiest!

cement to work properly really needs to be about 3 inches thick
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Old 04-06-2015, 09:17   #8
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Re: Cementing the watertank on a steel boat

I would say the bladder would be the best option, but make sure that you have the hull audiogaged so that you know how much of it has been eaten away... it is also good to do that every once in a while when the boat is out of the water. We did this on ours, and ended up cutting out a few major sections on our Kanter Yachts steel Spray 38. Best of luck!
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Old 04-06-2015, 10:10   #9
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Re: Cementing the watertank on a steel boat

How about Ospho? We've had good results.
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Old 04-06-2015, 10:11   #10
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Re: Cementing the watertank on a steel boat

Flexible plastic bladder is the simple way to go. Just make sure the hull is sound and properly recoated in the tank area.
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Old 04-06-2015, 10:35   #11
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Re: Cementing the watertank on a steel boat

yep -cut out the steel. that's what 's so good about steel - especially on a 29' boat - it's not a lot of steel we're talking about here. get some good slitting disks on a real grinder, suit up and wear protection, and cut the side out of the boat. Wait - before you do that, make friends with a good welder. discuss it with him/her. then get rid of the crap steel and put back good, clean, new steel - pre-coated, with the edges ground back for welding. Then you just have to stripe coat the welds on the inside after replacing it.

I just did this on my own boat. it's dirty, loud and tough work, but you get good access, and the result is as good or better than the day the boat was built.
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Old 04-06-2015, 11:42   #12
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Re: Cementing the watertank on a steel boat

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Originally Posted by NSboatman View Post
yep -cut out the steel. that's what 's so good about steel - especially on a 29' boat - it's not a lot of steel we're talking about here. get some good slitting disks on a real grinder, suit up and wear protection, and cut the side out of the boat. Wait - before you do that, make friends with a good welder. discuss it with him/her. then get rid of the crap steel and put back good, clean, new steel - pre-coated, with the edges ground back for welding. Then you just have to stripe coat the welds on the inside after replacing it.

I just did this on my own boat. it's dirty, loud and tough work, but you get good access, and the result is as good or better than the day the boat was built.
Whoa whoa, whoa, You need to see how badly the hull is corroded before taking such drastic measures. I have not seen it nor neither have you. Lots of folks here on CF make drastic statements with-out having the facts. If in doubt, get a professionals opinion, maybe more.
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Old 04-06-2015, 12:40   #13
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Re: Cementing the watertank on a steel boat

This is really frustrating... I had a big reply ready, clicked submit, and it was all gone... Anyway, here it goes again. I do appologise if any of the comments below sound blunt or short.

I'm not at all concerned about the integrity of the hull. It's 5mm thick, and since steel rusts at a 1-10mm ratio, 5mm of steel will turn into 50mm of rust before rusting through. The rustbubbles are MAYBE pea-size.

A bladder wouldn't work due to the really, really awkward shape of the tank. I can jam one in, but it won't make it till the end of the year, guaranteed.

Cutting the side out, making the tank look like new again inside and replacing it with new steel is simply way too much work at this point of the refit. For one that'd still mean cutting it out, grit-blasting, re-coating, re-welding, coating the welds from the inside, re-doing paint on the outside,... Secondly it's an old classic boat, with very curvy lines. That section of hull has 3 curves in alternating directions and all with different angles. Idk when y'all last tried bending 5mm thick steel last, but that's going to be a ***** to do. Not even to mention total cost of such a little project.

Cementwash is supposed to stop the rusting process. It's been used historically for decades to slap on rusted objects to stop the rust. It also sticks better to rust (rebar even HAS to be rusted prior to use). What I'm worried about is it coming undone from the side of the tank or cracking, letting water through and restarting the rust process. It also doesn't have to be inches thick, that would mean it will need reinforcing, pouring, and a wooden "casket" to poor the stuff into. Not only does that sound like a ridiculous amount of work for what it is, it would also add waaaaay too much weight. Imagine the 'ol guys doing that to their massive tanks . Anyway, the whole point is to make a cement sludge that you rub on in very thin layers, much like paint (5 layers the first go-around) to add up to 1-2mm thickness. Then you give it another layer every now and again.

Honestly, I had given up on the watertank alltogether before I heard about the cementwash thing. As I might have mentioned before, at this point in the refit I simply don't have the time or resources to tackle such a "major" project. I'm looking for something one guy can pull off in a long weekend an costs less than a boatbuck. If that's not at all possible, I'll cut my losses and the top of the tank to use it as storage for waterbottles. My future self will surely find time and resources to make the thing look brand-spankin'-new again, and I've used the bottle/jerrycan approach for many years living in campervans.
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Old 04-06-2015, 13:00   #14
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Re: Cementing the watertank on a steel boat

You're right that cement wash has been used for ages, and it is an effective finish for steel water tanks. Tom Colvin recommends it, and he designed and built scores of steel boats.

I have no idea how the cement will perform over your bitumen. I'd suggest you pressure wash and then apply the cement wash over a small section of the tank that has the bitumen on it. Let it cure and see how durable it is. If it sticks to the bitumen I'd second the suggestions to phosphoric acid etch the tank to remove the rust, then use the cement.

Another possible idea is to coat the tank with coal tar epoxy, again after acid etching. Coal tar epoxy has the unusual ability to adhere well to oily surfaces like your bitumen. I DON'T KNOW IF COAL TAR IS OK FOR USE IN CONTACT WITH DRINKING WATER. For sure you'd need to check that.

Good luck with it.
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Old 04-06-2015, 13:46   #15
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Re: Cementing the watertank on a steel boat

Both coal and tar sound horrible, not sure 'bout that .

In any case, I've looked into it for different uses (it's the holy grail for paint imho), and it's impossible to get due to the eco-warriors nowadays.
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