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aquanut
February 15th, 2002, 04:33 PM
Aside from all the pro's and cons of repainting tanks, (yes, I've heard them all), has anyone repainted their own steel tanks with any success?
Can anyone recomend the proper procedures, or recomend what type of facility that will repaint scuba tanks?
Any ideas would be appreciated.

t'anks alot! :)

FredT
February 15th, 2002, 06:36 PM
most steels are galvanized under paint, and any heavy thickness paint should be removed before visuals or hydro are performed. This is especially true of vinyl or epoxy coated tanks!

Repaint with a good cold galvanizing compound ONLY. ZRC is about the best available in North America. Although some other "cold galv" formula paints are available ZRC stands head and shoulders above them in salt water applications.

Rick Murchison
February 16th, 2002, 02:34 AM
I stripped, cleaned and repainted some OMS 85's this fall after finding a rust spot under the paint on one of the tanks. I repainted with clear epoxy. That may not be the best paint, but so far it's working fine, and now no rust can form without my seeing it.
Rick

Waterlover
February 16th, 2002, 08:47 PM
Rick,
How did you strip the paint layers from the oms tanks.I have 2 98's that the gray paint will just rub off.OMS won't give me any details but will paint them for $35 ea plus shipping both ways. Did you sand blast at all? TIA Ken

Rick Murchison
February 16th, 2002, 09:44 PM
Originally posted by Waterlover
Rick,
How did you strip the paint layers from the oms tanks.I have 2 98's that the gray paint will just rub off.OMS won't give me any details but will paint them for $35 ea plus shipping both ways. Did you sand blast at all? TIA Ken
What a royal pain in the behind stripping those things was. I used two different strippers, razor blades, sandpaper, wire brushes and hours and hours of good old elbow grease. Had I known how hard it would be before I started I might have settled for a spot repair, but I'm glad I went ahead and did the whole job - I have a whole lot more confidence that I won't get any undetected rust now.
Rick

Campana
February 17th, 2002, 11:27 PM
I like the old kind of "Strip Ease" (sp?). You gloop it on thick and leave it alone for a long time (30 minutes or longer). Then rinse it off, and hit it again. I think it's basically lye. Anyway, it'll bubble the paint up nicely, and you'll see if there's a nice galvanized finish under that old paint. Don't get in a hurry, don't start scraping or sanding until the chemical does it's job. If you have to, go in the house and eat lunch. Just don't get impatient.

If not galvanized, spray galvanize the bare, dry tank, and then paint it with a good paint like Rustolium. Fred T remembers the name of the best spray on galvanizing, but I don't remember it. I've got one I painted with John Deere Green paint and it looks OK. It's not as rugged as galvanized, though, and if you uncover galvanization, leave it alone, don't repaint.

I stripped two 72's that had a thick plastic coating on them. I think they were US Divers tanks from the 60's. The galvanized finish under that crap was the nicest I've ever seen. I shined em up, got em hydro'd and now they're a nifty set of doubles.

Rick Murchison
February 18th, 2002, 12:00 AM
Originally posted by Campana
I like the old kind of "Strip Ease" (sp?). You gloop it on thick and leave it alone for a long time (30 minutes or longer). Then rinse it off, and hit it again. I think it's basically lye. Anyway, it'll bubble the paint up nicely,
That's one of the strippers I used, and that's what I expected to happen, as I'd used it before stripping propellers on airplanes. But whatever OMS uses doesn't bubble up worth a flip! Tried some stronger stuff, too, which was a little more successful, but it still took a bunch of plain old hard scrubbing, scraping, sanding and brushing to get 'em clean.
Good luck!
Rick

FredT
February 18th, 2002, 12:28 PM
JProsser99 wrote on 4/25/97 rec.scuba.equipment:
>
> Seem to have missed the original metal tank coatings post which I did not
> see listed now. Looks like I missed a good post: can someone forward me a
> copy of it? Thanx! (jay’s address removed)

It's available on Deja news archive.

HTTP:\\www.dejanews.com

but you _asked_ for it so here goes.

-------------Start quoted text--------------------

Carl Heinzl wrote:
>
> >Now Carl - what's wrong with OMS tank? I was about to buy one tomorrow.
> >Seriously!
>
> I too was going to buy some OMS tanks before I found out all this. Ok
> this is from memory as some of this stuff is till put away in boxes.
>
> 1) Their buoyancy characteristics aren't as good as pressed steel.
>
> 2) I forget what type of galvanizing they do, I think it is sprayed on,
> but it is NOT hot dipped galvanized like pressed steel (and that is a
> big one).
>
> There was more but I don't remember it off the top of my head. Pick up
> tanks that are *hot dipped* galvanized though.
>
> Perhaps others can expound on this more fully than I.
>
> -Carl-

Carl,

I don't have an OMS tank and know of no incidents of coating failure (or success for that matter) with them, but I am relatively familiar with coating technology.

There are 3 commonly used methods for galvanizing in production operations. Normally flame spray is not one of them, but it may be used if there was a concern for the heat treatment of the tank. The normal methods and some of the differences are: (all dimensions in inches)

1. Hot Dip
This process involves mechanically or chemically cleaning the object to be coated to bare steel, sometimes preheating the object, and then dipping the object into a tank of molten zinc. The zinc will form a metallurgical bond with the steel similar to a brazed joint where a small percentage of each metal goes into solution in the other forming a bond at the atomic level. The amount of coating remaining on the object is dependent on zinc pot temperature, withdrawal rate and any mechanical removal done as the hot coated object is removed from the tank. Normal coating thickness is often in excess of 0.005". Air knives and mechanical wipers are used on some products to limit film thickness in critical areas. A multiple dip process can obtain heavier coatings in excess of 0.030”.

++Hot Dip normally provides the best adhesion, heaviest film thickness, and least pinholes in the coating of any method.
++ Pinholes tend self-seal when in contact with water.
+Can be done in an automated process on a production line with minimum labor involvement.
+Process quality control is easy to monitor and has relatively wide acceptable levels for the critical variables.
-Energy intensive as the entire object must be heated to above the melting point of zinc and the molten zinc must normally be maintained at plating temp round the clock to provide production throughput.
-Expensive USA EPA and OSHA requirements for zinc fume scrubbing and worker contact can run up the costs significantly over previous years.
-Effectively prohibits any heat treatment step requiring soaking at or below the zinc pot temperature from being effective. Normal realities involved in heat treatment of steel will usually prohibit engineers from incorporating the galvanizing process into the middle of a heat treatment process following the hot forming of the tank itself since beginning and end of day in process work will have different time constants. This limits material choices.
-(For the manufacturer) thin films are often difficult to maintain with this process. This uses more zinc and thus adds cost to the product. This heavier minimum coating I consider a + for scuba tank owners. Makes it much harder to get a tank off spec on the thin side.:-)

2. Electrogalvanized
This is simply an application of zinc to the surface of the steel by the electroplating process. The object to be coated is etched to remove any surface contamination and immersed in a chemical bath with an applied voltage. The coating drops out of solution on the surface to be coated. The bond is normally at the metal crystal and/or surface irregularity level. Normally used where the required coating thickness is measured in parts of a thousandth of an inch to 0.002". Heavier coatings can be obtained at longer bath dwell times but thick (.005 and up) single coat electroplate of _any_ metal has a tendency to bond failure and will often chip off during localized impact. Coating will preferentially be deposited on sharp outside corners and be thinner in valleys. This may be a problem at tank necks. The process is relatively cold and does not limit heat treatment choices. Coating variables include bath chemical concentrations, dwell time, current density per square inch, temperature, and driving voltage.

+Well understood and developed technology.
+(For manufacturer) Easy to maintain constant coating thickness with minimal material usage.
+Can be set up as part of a production line with limited impact to start and end of day work in progress.
+No constraints on heat treatment/wider material selection.
-More variables and relatively narrower acceptable bandwidth for quality control purposes. Monitoring equipment is more maintenance sensitive than that needed for HD.
-Chemical baths often require treatment as hazardous waste during disposal driving up costs.
-Coating can have unbonded areas not immediately visible on inspection and pinholes do not normally self heal as rapidly as HD, if at all.
-Chemical baths require a similar level of US OSHA and EPA safety hardware and pollution control as HD.

3. Impact coating.
This process involves placing zinc (or other metal powder) in a tumbler along with the parts to be coated and an appropriate transfer medium (ceramic chips, steel balls, etc.) if required and running the tumbler for several minutes up to several hours or days. The coating is applied in a _very_ thin coat by being cold welded to the object by repeated impacts as the parts and transfer media roll around inside the tumbler drum. This process can also provide a limited deburring action. Often used with nuts, bolts and other small solid parts.

+Minimal material usage as unapplied powder and transfer media can be reused batch to batch.
+Minimal energy consumption.
+Time and transfer media wear are the only variables not totally controlled up front. Effectively no problem for quality control.
-Only relatively thin coatings <~0.0005" can be applied.
-Applicable to batch processing only.
-Difficult with scuba tank sized objects.


4. Flame spray or metal spray process:
The fourth type of coating is relatively rare in production but has gained widespread use in the maintenance industry for building up worn shafts at seal and bearing points and applying expensive protective coatings or hard surfacing at known wear points on otherwise lower cost items. Shafts and electrical contacts are two applications that come immediately to mind. I see no reason why a scuba cylinder _couldn't_ be coated this way, and a couple of reasons why it would appeal to the manufacturer. As far as the process path goes normally the material to be coated is mechanically cleaned to near bare metal at the start of the process. The process itself involves an oxy-acetylene, oxy-hydrogen, or other high temp torch impacting the surface to be plated and adjusted to provide a slightly reducing (oxygen starved) flame. The metal to be plated onto the surface is powdered and introduced into the base of this flame. Any surface oxidation on the part is removed by reducing any metal oxides on the surface back into the parent metal as the excess fuel in the torch consumes the oxygen from the metal oxides. It's possible to melt or soften the very surface of most metals without transferring much heat to the core of the product by controlling the torch temperature, flame velocity, and time the flame is in contact with any spot on the object to be coated. The coating metal powder is melted by the torch between the introduction point and when it contacts the softened or melted surface of the object and forms a good atomic level bond with it. Coating thickness is controlled by the speed of the torch, amount of powdered metal introduced, and torch path. Thickness can be as low as tenths of a mil (0.0001") up to several inches depending on the material deposited and number of passes involved. The variables in a production operation would normally be tracked by automated machinery and include powder particle size, fuel/O2 mix ratios, flame velocity, distance to surface being coated, powder feed rate, object temperature and cooling, etc. An example of an uncontrolled variable would be any transient jamming in the powder feed tubing. Provided the quality control is tight enough, and the coating is thick enough, a metal-sprayed zinc coating should be about the same as a hot dipped coating galvanized one. The greater number of critical variables in the process and the natural stinginess of production foremen (must minimize costs you know) combine to create a greater potential for coating failure in the field than in HD.

++ Good control of coating thickness and thus of costs.
++ Minimal USA OSHA and EPA requirements since molten metals and hazardous chemicals are not obviously involved. Fume collection on a relatively small flame spray booth would be about it. No, or minimal, hazardous waste since unconsumed powder may be reused or sold as a by-product to impact or electro-platers and die casters.
+ Frees up heat treat and material choices since sidewall core temperature can be controlled to _any_ reasonable point. It should be possible to limit wall core temp to less than 400F during the process. This will allow use of almost any High Strength Low Alloy or alloy steel for tank construction from a heat treatment point of view.
-Complicated quality and process controls are required to provide an acceptable product. Process transients can produce thin spots in an otherwise good coating without being immediately obvious.

The last item is the big kicker! Top management commitment to quality control is MANDATORY for this type of coating to be consistent and suitable for repeated immersion in seawater. Thin or spotty coatings on a few tanks will kill their reputation with the tech crowd. From what you said earlier it sounds like this has already happened. The tech crowd also tend to be a vocal lot with well-formed and entrenched opinions and tend to pass these on to others not of the tech persuasion. Considering that the techies are often opinion leaders in this game it sounds like OMS may have stepped in it big time!

IMNSHO If OMS has developed this reputation the only saving grace it has open to it would be to match Luxfer's neck crack tank replacement policy with their coatings. As it was explained to me if any Luxfer tank fails a VIP due to neck cracks they will replace it, period. No limitation on number of owners or age of the tank. Ship it to them with the PSI inspector’s paperwork and they'll ship back a new tank! If OMS
unconditionally stood by their coatings to the same extent no diver would think twice about buying one. If he got a bad one it would be replaced with a new one. No (hard) questions asked. No proof of purchase or other legal nastiness required. For the price of the postage to get the tank back to them with a VIP inspection report a new one would appear at your door in less than a month!

FT

BTW Not having ever personally dealt with them I have no idea what their warranty currently is.

neil
February 18th, 2002, 03:43 PM
FredT,

About 6 years ago I had my steel 95 flame sprayed (they called it "metallizing") and then painted. There has been no problem since. The flame spray coating is actually quite rough by itself, but it gives great bite for a paint job.

Neil

FredT
February 18th, 2002, 05:51 PM
DOT specs are very specific about scrapping rated pressure vessels exposed to high temperatures! A single accidental arc strike mark is enough to condemn a cylinder, even with no perceptable penetration into the wall.

I havn't checked metallizing, but it may be a similar case. In any a case a hydro immediately after the process is a good idea.

AS a rule of thumb with steels:

1. Spot treatment of a defective coating is preferable if 90% or more of the original coating is present.

2. Chemical coating removal is preferable to mechanical removal if at all possible. Nitric or hydrochloric acid will strip damaged galvanizing fast! (Goggles and LOTS of ventilation are needed here!) For slower and a bit safer stripping phosphoric acid is a better choice, not to mention that the resulting solution will "parkerize" the tank and make repeat rust less of a problem once the tank is cold galvanized to repair the coating.

A product called Exrust by Kano Labs out of Nashville is one of the easiest phosphoric products to both aquire and use here in the US. The easiest way to get a visual on how this works it to visualize a mixture of phosphoric acid and windex. It has enough detergent and surficant action to deal with thin oil films with minimum agitation, with enough concentration of phosphoric acid to do the job quickly without being very tissue agressive in the case of skin contact. It's also effective in cleaning the inside of tanks with a light rust, and will stop any interior pitting in it's tracks when used in conjunctin with a ceramic media tumble operation.

3. Spot treatment is easily done with a grey "ink" eraser in
a power tool (think "drafting supplies" to find a source of these) to abraide topcoats and remove any surface rust to bare metal, touch coat with exrust to kill any pits, then topcoat with a good cold Galv. compound like ZRC. ZRC is also available now in an aerosol spray. CRC cold galv is good as a second choice with the Rustolem or equal a distant third choice.

FT

devjr
February 18th, 2002, 07:10 PM
In San Diego, there was an outfit that flame sprayed our war surplus, old tanks. No problemo, I've still got four of them lying around my locker. They didn't make any money on it, though; just kept the local divers happy. The shop foreman told me that the cleaning had to be meticulous, that there could not be so much as a fingerprint on the tank before using the plasma gun.

CRC zinc spray is a good overcoat, like you said.

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