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Sandblast metal to prep for welding... Bad idea???

Discussion in 'The Hokey Ass Message Board' started by Pir8Darryl, Feb 27, 2011.

  1. Pir8Darryl
    Joined: Jan 9, 2008
    Posts: 2,488


    My brother, who used to work as a welder, brings over a metal hatch lid for me to fix. It's 1/8" sheet and it's got a crack about 3" long at the hinge.

    Sure Bro, no problem, follow me out to the garage... Hit it with the sandblaster to strip the paint around the crack, and then carry it over to the welder.

    He stops me and asks WTF I'm thinking? Tells me you are NEVER supposed to weld sandblasted steel, as the sand embeds silicone crystals in the metal, and it'll cause the weld to be brittle and crack.

    Really? :confused:

    I always wipe the dust away with a rag before I weld, but I've never heard this before. Just to make him happy, I made a couple passes over the damaged area with a wheel, welded up the crack, and sent him on his way.

    Like I said, he worked as a welder for about a year [a few years back], so I'm thinking he might know something about what he's saying... But I've never heard of this before. I was told that sandblasting is a good thing because it works the metal up to allow better penetration... Never heard about silicone contamination.

    Any thoughts?
  2. wire brush or sand before welding, he is right. i noticed when i glass beaded aluminum it stays cleaner, i think because of the embedded glass.
  3. youngrodder1929
    Joined: May 28, 2006
    Posts: 416

    from Vancouver

    i wouldn't think so iv done it quitabit theres silicone in the welding wire any ways for anti oxidization
  4. Del Swanson
    Joined: Mar 27, 2008
    Posts: 708

    Del Swanson
    from Racine, WI

    I agree, sand or wire brush before welding.

  5. 53mercury
    Joined: Dec 2, 2010
    Posts: 95


    I've welded blasted frames, exhaust, and other steel and no issues. JME Mike
  6. driveability
    Joined: Feb 16, 2011
    Posts: 7

    from everson WA

    I read the problem is when welding aluminum.
  7. newsomtravis
    Joined: Jun 1, 2009
    Posts: 562

    from pville, ca

    as a welder i would say, wouldn`t be a problem, especially on eighth inch.....if your welder won`t penerate that........well, i donno what to say......
  8. Pir8Darryl
    Joined: Jan 9, 2008
    Posts: 2,488


    Same here... I've tubbed narrowed the frame on a race car, did a 4 door to 2 door concersion on a shoebox, and dozens of other projects... Had a couple small issues with my welding in the past, but never anything that I could [now] look back on and say "maybe it was silicone contamination".

    He worked as a welder building river barges, so he was working with 1/2" and 1" steel plate... I wonder if that has anything to do with it. AFAIK, he never welded aluminum.
  9. LBCD
    Joined: Oct 28, 2009
    Posts: 1,029


    aluminum makes sense because its both soft and porous, and it depends on the media being used too.
  10. my welds are better after a quick swipe with a brush after sandblasting. maybe it's just me, or my sand, or maybe my blaster.
  11. squirrel
    Joined: Sep 23, 2004
    Posts: 47,812


    silicone? silicon?
  12. Pir8Darryl
    Joined: Jan 9, 2008
    Posts: 2,488


    To-may-to... To-mah-to.... Po-tay-to... Po-tat-o.

    Maybe my signature line should read "I know what it is, I just can't spell it" :D
  13. havi
    Joined: Dec 30, 2008
    Posts: 1,875


    silicone causes fish eye in paint... silica(oops) is sand, no offense. We do heavy welding where I work, and the sandblasting is done first with no regard to worrying about silicon. I can understand there being an issue with aluminum as well, though.

    I'm not a pro welder, so I can't say firsthand, but just what I see at work. I believe they run the sander over the surface to smooth it out first, though.
    Last edited: Feb 27, 2011
  14. No Cents
    Joined: Feb 28, 2009
    Posts: 331

    No Cents

    Isn't glass made from SILICA ?
    I have never had any problems from doing this.
    I'm not a metallurgist but I have been a welder for over 35 years.
    Aluminum is a little different, It HAS to be VERY clean to weld properly.
    So I could see problems doing this to aluminum.
    Last edited: Feb 27, 2011
  15. AJofHollywood
    Joined: Oct 3, 2008
    Posts: 640


    I read it other places too and in books, not to weld sandblasted metal. So I took it to heart and sanded the areas I was to welded (on the my frame) shiny just before jigging and welding.
  16. Pir8Darryl
    Joined: Jan 9, 2008
    Posts: 2,488


    Bump for the morning guys... I'm really interested in knowing if this is true.
    So far only one guy who confirms it based on his reading it in a couple places. [no dis-respect AJ]
    If anyone can provide first hand knowledge or a good scientific explination, please do so.
  17. 283john
    Joined: Nov 17, 2008
    Posts: 814


    why didn't he stop you before you blasted?
  18. dabirdguy
    Joined: Jun 23, 2005
    Posts: 2,404

    Member Emeritus

    Sand paper would imbed and leave traces of sand in the metal as well. If Silica was an issue, sanding would not work either.
  19. Pir8Darryl
    Joined: Jan 9, 2008
    Posts: 2,488


    He knew I was blasting it. He assumed I was going to wheel it before I welded it.
  20. olddrags
    Joined: Oct 19, 2006
    Posts: 476

    from ky

    I'm with dabirdguy on this one! I also think if contamination was an issue the heat from welding would eliminate the contaminates. Been welding over sandblasted metals for 40+yrs aint had a problem yet.....
  21. ntxcustoms
    Joined: Nov 10, 2005
    Posts: 910

    from dfw

    Yeah I would think that anything you use to clean the surface (talking grinding stones or paper) would leave traces behind. I personally have never had an issue with welding freshly blasted metal, can't speak for others. I was taught welding at the San Marcos job corp starting with gas on up to tig. I was offered a position at some ship yards constructing large naval boats and barges. Almost all the plate they had was blasted and primed immediately with no issues to welding...
  22. CutawayAl
    Joined: Aug 3, 2009
    Posts: 2,144

    from MI

    I learned from working with magnesium and dealing with corrosion that media blasting, sanding, and machining all leave minerals on the surface. That is so even when the part seems perfectly clean with no contamination or particles visible. Simply touching the part with a steel tool deposits material, just like marking with a crayon.
  23. Pir8Darryl
    Joined: Jan 9, 2008
    Posts: 2,488


    True enough... But is there some spacific property of SILICA that would have any effect on steel?
  24. i'm not saying you can't weld sandblasted material, [i sand blast everything] i'm saying the welds are better if they are cleaned up prior to welding. i never read it anywhere, i just noticed it. i can't believe i'm the only one. i do believe driving sand into metal at hurricane force speeds that's capable of shattering the grains of sand doesn't also embed tiny pieces into the surface.
  25. Stefan T
    Joined: Sep 15, 2008
    Posts: 2,166

    Stefan T
    from Sweden

    I have seen problems with weld sandblasted steel one time

    Smal spot welds cracked but after little grinding it didn't
  26. Streetwerkz
    Joined: Oct 1, 2008
    Posts: 718


    We blast everything befor we weld it.
    The key is to blast with a media that does not impinge itself in the metal, or leave any part of it behind. for this reason we use aluminum oxide on steel, stainless, aluminum with no problems. the alum oxide makes contact with the part & explodes on the surface to remove contaminates, not leaving any by product on or in the panel. We will also blow the panel down & wipe it with denatured alcohol for extra c.y.a.

    I'm sure there's other ways to do it, just one of the ways that has werked for us
  27. gimpyshotrods
    Joined: May 20, 2009
    Posts: 17,904


    I have been blasting (Aluminum oxide and coal slag), welding right afterward, 26ga to 3", for over 20 years, certified, with no problems whatsoever. Some of my welds are currently helping hold up the tallest building in San Francisco.

    If you are really blasting with sand, I seriously hope that you are taking every precaution to keep the dust out of the air that you and the people around you are breathing both during and after breathing. It is extremely dangerous to breathe silica dust. I'd recommend a different blasting media.

    Most other blasting media I have seen advertises that it is 99% silica free, which is to say, it has less silica in it that the welding wire you just welded it with.

    The silica is a much lighter material then the welding wire, and will float to the top of the weld puddle, or vaporize into dust. You will see this regularly with ER70S-6, which is a pretty common general purpose wire. The little glossy deposits on the top of the bead here-and-there are glass, which is made from silica.
  28. pitman
    Joined: May 14, 2006
    Posts: 5,030


    Wikipedia gets into this pretty well. There are some "electrical" layered steel alloys w/about 3% silicon present as an alloying element. This has the better magnetic properties for transformers and such it seems. Car sheet metal is not this type however. The materials guys call the surface/structure a "morph"ology, so we can imagine the blasted surface as highly tweeked, and would melt back when brought into a weld-zone's energy state. Silica is SiO2, or oxidized Silicon. Silicon is the reduced form (like AL) so the oxygens were driven off. As to prep before welding, grinding would flatten the high strands of blasted, but I'd guess we'd have to test a couple of samples to see if it mattered or improved the results
    Last edited: Feb 28, 2011
  29. CutawayAl
    Joined: Aug 3, 2009
    Posts: 2,144

    from MI

    I have no idea. While I have noticed that blasted meterial welds differently, I have no first-hand knowledge as to why. I always assumed that fine dust/deposits left behind by blasting was the cause, but I don't know.

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