External rust, stripping & painting steel tanks

[AzAtty]

For what it's worth, Faber tanks use a "spray coating" galvanizing process. Cold spray galvanizing doesn't protect steel as well as a "hot dip" process. So Faber tanks are a bit more susceptible to seawater corrosion than hot dipped tanks.

For those who don't understand what galvanizing is, zinc, which is highly resistant to corrosion, is electrochemically bonded to the steel tank. The zinc "layer" varies in thickness, depending on the application and the manufacturer's specification, and the thickness of the zinc layer determines the service life of the equipment. For instance, a 5 micron zinc layer gives the galvanized item a service life of approximately 100 years; "service life" being defined as the time to a five percent rusting of the steel surface.

If the tank has already begun rusting, the damage is already done. Merely grinding off the rust and painting the tank won't solve the problem. Indeed, taking a grinder to the tank guarantees that you'll cause further damage to the zinc layer. You're also scraping off layers of steel, which I don't think is such a great idea for a high pressure cylinder.

Applying primer and a paint coat on the metal will not adequately protect the tank, even if you could get the paint to adhere. The other problem is that metal oxidization occurs on a microscopic level, and removing the visible rust does not guarantee that you've eliminated the oxidization problem. Slapping a coat of paint on the cylinder may actually hide a portion of the tank where oxidization is continuing.

If you really want to fix the problem, I would suggest that you check around for companies that provide post-fabrication galvanizing and talk to them about how to clean the tank and reapply the zinc.

 

[Rick Murchison]

First, I tried the boot-on, boot-off management scenario, but eventually just gave up and now have a wooden rack to stand the tanks in for storage - the boots... are gone.
I ended up stripping my Fabers (OMS's) completely (this is a major job as the paint they use is really hard to get off when it isn't peeling on its own), and repainting with clear epoxy. Now, if any rust starts I can see it immediately and take action.
Rick

 

[MechDiver]

Rick Murchison once bubbled...
First, I tried the boot-on, boot-off management scenario, but eventually just gave up and now have a wooden rack to stand the tanks in for storage - the boots... are gone.
I ended up stripping my Fabers (OMS's) completely (this is a major job as the paint they use is really hard to get off when it isn't peeling on its own), and repainting with clear epoxy. Now, if any rust starts I can see it immediately and take action.
Rick

Rick,

Could you post details of what you did and products used? Or PM me with info please.

MD

 

[DocRCH]

I have not seen where "hot dip" is better than being sprayed. I have seen where the heat of a hot dip may be bad for the steel in terms of its strength. When a metal is made, there are grains in it. Heating a metal such as steel can disrupt these grains change its physical properties.

The problem with scuba tanks is wear against something else. The steel is coated with an organic substance (e.g. epoxy, urethane). Any of these can be worn off. Also, the gaivinizing can be worn off also. Nothing is impervious to wear.

As far as putting something over the damaged areas, look at www.zrcworldwide.com. They have a number of products like www.zrcworldwide.com/galds.htm

Hope this helps, Robert :doctor:

 

[Rick Murchison]

MechDiver once bubbled...


Rick,

Could you post details of what you did and products used? Or PM me with info please.

MD

Well, when I discovered rust hidden under the paint I decided to strip the tanks - had I known then what I know now I don't know whether I would do it again... what I thought was going to take about two hours took nearly two weeks of meticulous work in my spare time getting off all the paint without damaging the zinc underneath. I used a commercial paint stripper, which had barely enough effect to soften the paint to the point where I could scrape it off a tiny strip at the time with a heavy duty razorblade. I came behind that with some light sanding (zinc is soft and you don't want to remove that - just the paint) and finsihed cleaning with a rotary wire brush. Once cleaned I had 'em hydro'd as a precaution to make sure that (1) the little bit of rust I'd removed was in fact a "little bit" and didn't affect the tank's structural integrity and (2) to make sure I hadn't hurt 'em during the stripping process.
After I got them back from hydro I gave 'em another bath with a mild vinegar solution, rinse with distilled water & dried, then painted with clear epoxy - the old fashioned kind that you mix the hardener in and paint quick before it sets. (to paint them I put some old out-of-service valves in 'em and hung 'em up by those).
Worked out great - I can now see the surface of the tanks through the epoxy, and since it is epoxy it's super easy to fix any dings. So far this coating has been totally successful at preventing any further rust - I've been diving 'em about 14 months since I did it.
Rick

 

[ScubaJorgen]

Zinc painting
This weekend I asked a local paint specialist. He could sell me zinc paint, though he really advised to sand blast the metal first. That is usually the procedure for applying zinc paint. Just grinding and zinc painting is not sufficient, leaves rust underneath. :hour: Fred, are you still attending this thread?

Epoxy
He advised an expoxy. Paint is micro porous. No protection for rust. Epoxy is not porous.

Since I am not able to sand blast, I am going to paint my tank with epoxy. I'v removed the original paint and rust from underneath the boot by grinding. I grinded the remaining original paint. It is really stuck very well (does not come off), so I leave it there (saves me the Rick-labour ) Since I am usually diving in fresh water I hope this is sufficient.

Grinding tanks
I do not think grinding the tank is much an compromis to safety: before you've grinded off one micron of steel, your elbows are worn off. I think recent tanks are thicker than, say, 10-20 years ago (according to the weight difference between my old and my new 12 l tank). My old tank still passes the hydro's. So I think recent tanks are well over-dimensioned. :bonk: Any comments to this statement?

Zinc and tanks
I wondered if my Faber tank was galvanised. As AzAtty bubbled, zinc works electrochemical. The zinc corrodes (redox reaction), the surrounding steel does not. On ships they mount pieces of zinc at regular intervals on the hull. The intervals are larger than a tanks length. Zinc being present prevents steel from corroding. I would expect my tank not to corrode if there was any zinc left on it. :idea: Maybe I should mount a piece of zinc nearby the valve, making good electrical contact to the bare metal of the valve.....

 

[akforsyth]

Anyone done this before? Have any tips/tricks? Thinking about painting a couple of steels...Make them look purdy again...

 

[Aquamaniac]

They look a whole lot "purdier" in a dull galv coat.

Paint is for cars.

 

[cudachaser]

Around 20 years someone here in Florida got the idea to paint tanks with exotic designs and colors...and market his product!

His process included baking the enamal at 350d much like you do when repainting a car. This heating altered the characteristics of the Aluminum significantly. The tanks would rupture during filling. I recall there were deaths.

Many dive shops recall this incident and will probably not fill your tanks if you paint them

 

[Bob3]

Nice shiny new looking stuff makes ya look like a newbie, I'd just roll 'em around in a gravel parking lot to even out the scratches.
:tease: