Sand Blasting of Aluminum Cylinders

[spectrum]

I am picking up a few AL80s that are "not too pretty" as the owner described them.

I'd just as soon get them down to bare aluminum with a uniform appearance and was thinking of sandblasting them.

I found some references in my search, none of them negative. Other than doing it with a light touch to remove only the paint and the slightest amount of metal are there any suggestions, regulations or personal experiences I want to know about?

Thanks,
Pete

 

[mwhities]

I was told you don't sand blast al tanks. you need to bead them. Maybe the sand is too course and will eat away to much.

I'm sure someone more knowledgeable will chime in.

Michael

 

[fweber]

O.K. I'm sure Luis can chime in with a discussion of metalurgical grain structures and the results of impact, etc.

But, in my simplified gas shack terms. The only "recommended" way, and the process that luxfer and Catalina use, and that we use for both aluminum and steel, is "sling type shot blasting". It's a large steel cabinet with essentially paddle wheels that throw the steel shot at the cylinder as it rotates and slowly passes before the shot wheel.

The reasons for not "sand blasting" to the best of my ability to remember and explain are two fold.

1. Sand blasting, or any abrasive media, is going to remove material and so require measuring that remaining sidewall thickness still exceeds minimum standards. Something, many hydro stations are not equiped to do. Apparently, steel shot with it's larger and smoother surface is less likely to remove parent material.

2. And this part, I'm not the authority on so don't quote me. But, the air/gas velocities required to move the media in a normal air/gas blasting aparatus are significantly higher than those reached using the sling method. Apparently, this leeds to a higher likelyhood of peening or work hardening the surface structure of the cylinder. The sling method although using a heavier media is propelling it at a much lower velocity. Which apparently has much less effect on the surface of the cylinder.

Luis, have I explained this properly?

 

[spectrum]

O.K. I'm sure Luis can chime in with a discussion of metalurgical grain structures and the results of impact, etc.

I'm sure he will, we're diving this afternoon.

Thanks for your input.

Pete

 

[Luis H]

From Luxfer

Here is the web site link:
Luxfer: Technical Bulletins: August 9, 2006




January 16, 2007
Paint removal and repainting of Luxfer all-metal and composite cylinders made from 6061 aluminum alloy (L6X®)1

1. Preparation

a) All-metal Luxfer 6061 aluminum alloy (L6X®) cylinders—To remove paint from an all-metal cylinder and prepare the surface for repainting, blast with plastic, glass or other soft media just long enough to remove most of the old paint. Do not blast with sand, steel, grit or aluminum oxide media, which are too abrasive. Heavy or improper blasting with hard or overly abrasive media removes metal as well as paint from the cylinder, which can thin the cylinder wall and make the cylinder unsafe to fill or use. Improper blasting also obliterates stamped markings and damages the outer surface of the cylinder.

Never use corrosive, caustic or acidic paint strippers or solvents to remove paint. Never use burning or torching techniques. Do not use knives or other sharp instruments to remove labels.

Once paint has been removed, use a neutral, mild, water-based detergent to wash the cylinder. Rinse the cylinder well and thoroughly dry it at a metal temperature not exceeding 160 degrees F (70 degrees C). Carefully inspect the cylinder surface for damage and corrosion in accordance with all applicable regulations. If the cylinder passes the inspection, it is ready for repainting.

b) Composite cylinders—Do not attempt to remove intact old paint from composite cylinders by blasting or any other means that may damage the cylinder exterior. Lightly rub the surface with fine-grit emery paper to prepare it for repainting. If the surface is dirty, clean it with a neutral, mild, water-based detergent, then rinse it well and dry it thoroughly.
Never use corrosive, caustic or acidic paint strippers or solvents to remove paint or to clean composite cylinders. Never use burning or torching techniques. Do not use knives or other sharp instruments to remove labels.
Once the outer surface of the cylinder is clean, inspect it for damage in accordance with all applicable regulations. If the cylinder passes the inspection, it is ready for repainting.

c) All cylinder types—Do not use the above preparatory processes on the internal surface and thread area of any Luxfer cylinder. Different processes (not covered by this document) should be used to clean threads and internal surfaces.
While preparing a cylinder exterior for painting, take care not to damage the valve connection area or cylinder threads. Do not allow any process materials (e.g., cleaning agents, blast media, grit, etc.) to contaminate the threads and inside of the cylinder.

2. Painting

a) All-metal Luxfer 6061 aluminum alloy (L6X®) cylinders—Do not use acidic or caustic paints that may damage the cylinder exterior. You may use any neutral, air-drying or aliphatic, water-based or solvent-based liquid paint that is compatible with aluminum. You may also use neutral, aluminum-compatible powder coatings as long as curing temperatures and exposure times do not exceed those recommended by Luxfer. Please note that different aluminum alloys have different maximum-temperature specifications; for example, for Luxfer 6061-alloy (L6X®) cylinders, the maximum metal temperature must not exceed 350 degrees F (175 degrees C). Exposure times at various temperatures vary according to alloy. Please contact Luxfer for time/temperature information about the particular cylinder and alloy that you intend to repaint using powder coatings. Cylinders subjected to temperatures and exposure times exceeding Luxfer recommendations or heated to unknown temperatures must pass hydrostatic tests before being filled or used.

b) Composite cylinders—Use flame-retardant, air-drying or aliphatic, water-based or solvent-based epoxy or polyurethane liquid paints on Luxfer composite cylinders. Do not heat cylinders to speed up curing or drying of paint. Do not use powder coatings or other coatings that must be heated or baked. Never expose composite cylinders to heat exceeding 180 degrees F (82 degrees C).
Do not paint over cylinder labels. Mask labels before painting and make sure that they remain legible. (Note: Color coding cylinders to identify gas contents or usage does not eliminate the regulatory requirement for legible, properly configured labels.)

c) All cylinder types—Do not allow paint to contaminate the threads, valve connection area or internal surface of any Luxfer cylinder.

 

[Jimmer]

Luis, what would you think of soda blasting a cylinder? Seems to me it would do a good job of removing the paint, without getting into the metal.

 

[fweber]

Thanks Luis,

Obviously Luxfer has changed their recommendations. Strange thing is, I've had a discussion with their service department about this very topic since that bulletin came out, and they didn't mention it. Hmmmm. Gotta love em.

 

[Luis H]

Luis, what would you think of soda blasting a cylinder? Seems to me it would do a good job of removing the paint, without getting into the metal.

Sorry, I am not familiar with that process.


I didn't spend too much time looking around in Luxfer's web site this time, I just did a quick search, but they seem to have a lot of useful information from what I recall seeing.

I don't deal much with aluminum tanks, most of my tanks are steel 72 (from the 60's and 70's). I also haven't actually worked on a dive shop or hydro facility in over 25 years. I do occasionally assist and consult with technical information (and vintage scuba equipment) several of my local dive shops. I have also being doing a bit of technical consulting for PSI / PCI (and expanding my knowledge by working with them).

 

[NorthWoodsDiver]

Blasting can be performed with a variety of media including glass beads, walnut shells, sands, plastic beads, and dozens of other medias.

I would think you have a couple options with this.

You could take the cylinders to a machine shop with a large old style lathe and you could use a 80 grit sand paper to remove the paint. of have the machinist remove .005 inches of material which should leave you with little or no paint and hopefully a machine surface.

Tanks are built to be pressurized to well over 3000psi so removing a layer of paint should remove about 5-7 thousanths and than another 2 thousanths should leave you with are beautiful finish but if you used the sand paper technique you would remove similar amounts of material but be left with a finish like luxfers brushed-no-coat finish.

The trick with that is finding a good machinist and spending the time doing the work. Machinists make good money and it would not be worth his time for what you would want to pay so you would need to find you favorite machinist friend to do the work for free.

Media blasting is another option and if done properly with a soft or course media if should not hurt anything. A good blasting professional will know what would work best but its likely gonna be spendy. look at spending up to $60 per cylinder for blasting.

Chemical cleaning would be an option also but I dont know what would work best but I have heard finger nail pollish can take paint off tanks over night but I dunno for sure.

The last option is the luxfer trade in/ refinish program. You can send your old cylinders to luxfer for reconditioning +hydro +VIP for about the same price as a new cylinder or you can trade in your cylinder to luxfer (this is assuming you have luxfer tanks cause I think they only take their own tanks back) for a discounted new cylinder.

No matter what you do unless you have friends in convenient places its gonna cost at least $50 per cylinder to recondition them.

 

[Stu S.]

If anyone mounts a scuba tank in an engine lathe, please post a picture for us. Let us know how this method works for you.