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Sand Blasting of Aluminum Cylinders

Discussion in 'Tanks, Valves & Bands' started by spectrum, Sep 21, 2007.

  1. Jimmer

    Jimmer Solo Diver

    # of Dives: 200 - 499
    Location: Brantford, Ontario
    2,933
    21
    Luis, Soda blasting is a pretty cool process. I know one guy locally that does it. They blast dry baking soda instead of some sort of abrasive. It removes paint and rust without causing any damage to the underlying surface. They use it a lot in older cars, so they dont damage the already corroded metal. It's also used to clean brick without damaging the brick. Blasting Soda - Toronto, London, Hamilton - Sodablast

    I have a lot of machining experience, and all I have to say about that is good luck. You'd remove a lot more than .005" just trying to get the cylinder square and round.
     
  2. spectrum

    spectrum Dive Bum Wannabe ScubaBoard Supporter

    # of Dives: 500 - 999
    Location: The Atlantic Northeast (Maine)
    11,387
    820
    I don't think they are cylindrically true enough to run a cutting tool without removing excessive metal in some spots.
     
  3. fweber

    fweber Solo Diver

    310
    0
    I KNOW they aren't cylindrical enough nor straight(unbowed) enough to run a cutting tool up and down them. But hey, it's the internet, and the advice you get is worth what you're paying for it.
     
  4. mfalco

    mfalco Contributor

    # of Dives: 100 - 199
    Location: Mashpee, MA (USA)
    698
    21

    Thay also do it to boats to remove bottom paint without damaging the fiberglass. I would assume that if it won't damage fiberglass, it won't damage AL.
     
  5. FunkyDiver

    FunkyDiver Solo Diver

    # of Dives: 50 - 99
    Location: NW Ohio
    110
    1
    I have a used tank that my brother picked up from a guy who no longer dives. It was kind of beat up looking. A friend of his works in the automotive industry and has access to a shot beader. Basically they shot beaded the tank and put a clear coat on it. It looks pretty good now, but not exactly as shiny as new.

    When I went to get it filled yesterday at the LDS, he took one look at it and asked me what was the deal with the coating on there. I told him what was done to it and he refused to fill it. He said that legally, Luxfer and Catalina are the only people allowed to do anything to a tank like that. He took out his PSI book and showed me all kinds of examples of incidents where tanks exploded while filling, and it was due to people either heat treating the tanks (baking the new paint, which weakened the metal structure), or they sand blasted the tanks to clean them and too much material was taken off. I know this LDS owner pretty well, and I'm fairly confident this isn't some sort of a scheme to get me to spend money in his shop. He is not like that, and has never tried to steer me away from (or towards) anything else just to spend money with him. He did show me another tank a guy just brought in, that had been sandblasted and painted by a paint shop. This tank had definite deep pitting that you could see under the paint. I can understand not wanting to fill that, because it is obviously had too much material blasted away.

    This tank was hydro'd and VIS inspected after the work on the finish. If it passes the Hydro, shouldn't it be OK to fill and use? From what I have seen so far in this post, apparently there is a difference between sand blasting and shot beading? If what I'm seeing is correct, shot beading should be OK? I would appreciate any info that I could share with the LDS owner that would let him know this tank should be OK. It would have to be something I could show him in writing probably that would ease his mind, but at this point, I don't think he will do it.

    Thanks!
     
  6. oxyhacker

    oxyhacker Contributor

    1,314
    33
    This is all a gray area, so there are no firm answers. If the tank passes hydro and visual it should be safe to use, but passing hydro does not automatically void the other requirements, like minimum wall thickness. So while many shops will accept a hydro as absolute proof that a tank is OK if there are indications that it has been overheated or has had metal removed, many others will not, and that is entirely their decision.
     
  7. DirtRider

    DirtRider Contributor

    # of Dives: 100 - 199
    Location: NW suburbs of Chicago IL
    105
    0
    I'm no authority on this but here is another idea... Airplanes are made almost exclusively of 6061-T6 alloy. On my experimental the tail surfaces are only .016" thick. This is pretty typical for any private plane accomodating 6 or less passengers and have a top speed of < 250mph. You do not find skin thicknesses exceeding .040". Aircraft paint shops will use walnut shells or chemical paint remover. I just so happened to take the chemical approach when repainting mine and it worked great. I do not understand why this would be detrimental to a scuba cylinder. I'm sure you could learn a lot about the metallurgy situation and why not to use certain abrasives (there are reasons involving corrosion) if you included some aviation research with your scuba research.
     
  8. roakey

    roakey Old, not bold diver ScubaBoard Supporter

    # of Dives: 500 - 999
    Location: Colorado Springs, CO
    3,580
    182
    Actually I have a friend that does this, not to sand down paint from cylinders, but to tumble them. Works great!

    Roak
     
  9. spectrum

    spectrum Dive Bum Wannabe ScubaBoard Supporter

    # of Dives: 500 - 999
    Location: The Atlantic Northeast (Maine)
    11,387
    820
    As an update I have since stripped the cylinders with chemical stripper. Getting the paint off was an easy process.

    Since they had seen salt water use with failing paint (combo of factory and recoat) they are fairly mottled in areas where salt water was trapped under loose paint. The corrosion is not deep, it's barely preceptible with the finger nail rub. It's more of a stain than material loss.

    I hand rubbed one with a gray Scotchbrite pad. This made the nice areas look nice and did little to change the bemished spots. From here on I will just let them develop patina.

    Based on this experieince I would never suggest that anyone paint an aluminum cylinder especially for salt water use. An broken finish is good while it lasts. After that it's just a salt trap.

    Pete
     
  10. Eris Lucan7*

    Eris Lucan7* Contributor

    362
    0
    Aluminium is a dificult material.When it where construction steal or Karbonium or

    Titanium i woud say it is not bad;but at Alu!?

    E.L.7*
     

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