Prepping the steel plate without sandblasting

Discussion in 'Metal Boat Building' started by RayThackeray, Apr 19, 2013.

  1. tazmann
    Joined: Aug 2005
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    tazmann Senior Member

    Tight thick mill scale is tough to get off even with a sandblaster. Vinegar will soften it a little . running your torch flame close over it will pop it loose also just be carefull not to heat the plate under
     
  2. Titirangi

    Titirangi Previous Member

    Try using a sodium bicarbonate blast, its a wet blast that produces no dust and the waste if the scale is magnet separated from the sodium can legally go into storm drains.

    I used the sodium blaster to remove 130yrs of paint, scale and cement splashes from very large forged wrought iron (puddle iron not mild steel) bank window bars, fence & gates panels in situ on a heritage & conservation job I tendered for located in the city centre.

    The iron came up bright finish so good we could see the fibre of iron, before priming.
    I used the same method on 6mm marine grade nickle steel plate and got the same bright finish, the surface flash rusted but the primer I used required a surface rust
     
  3. RayThackeray
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    RayThackeray Senior Member

    Er - what part of the title that says "Without Sandblasting" was unclear? :) :)

    We did the job. A lot of real estate, and after trying just about every suggestion except blasting, we kept simply going back to angle grinders and disks. Everything else failed to remove the mill scale beyond a square foot or so. At one point we had 12 volunteers in 4 teams on grinders. It made for a very rough steel surface, I'm confident it's good for the primer paint and after a few weeks on the water all still looks like new under the waterline. It's amazing how hard the mill scale is - even an angle grinder with hard abrasive disk and you can slide over it until you dig in, you learn a technique to cover a six" by 6" section in a very methodical way, then just move on to the next section...
     
  4. tomas
    Joined: Nov 2012
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    tomas Senior Member

    "12 volunteers" ?

    Your powers of persuasion are impressive.
    Congratulations on completing the job.
     
  5. Titirangi

    Titirangi Previous Member

    I give up, what part of the title was unclear.

    Please note my comment was about sodium blasting not sand blasting.
    Sodium blasting as I mentioned is wet, uses a water blaster, it's cheap, doesn't require shrouding the site or shed dust extractors and can even blast composite hulls without damage.

    Good to hear you got the job done anyway even if it was the hard way :)
     
  6. michael pierzga
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    michael pierzga Senior Member

    I see soda blasting occasionally. The cleanup is tedious. Sand blasting is the way to go for boat hulls.

    Enviromental rules are pretty tough here...pro blasting only and naturally the work site must be protected and cleaned up afterwards.

    Even with the extra cost of Pro contractors and toxic disposal its still the cheapest, best way to go.
     
  7. RayThackeray
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    RayThackeray Senior Member

    4 months and the coating still looks good.

    Well, the boat's in the water and I've been looking at the paint below the waterline about 4 feet. So far no bubbles or rust spots of any kind, I'm glad to report. The final basic technique used after much experimentation:

    1. Below waterline carbon steel hull treated by angle-grinders and elbow grease to remove the mill scale and rust and get a clean abraded steel finish. Eventually we used a mixture of 40 grit sanding pads and 4.5 inch grinding disks. Sanding proved to only do about a square foot before it just started polishing, the most effective was grinding disks.

    2. We rolled on three coats of 2-part epoxy primer paint.

    3. Then we rolled on three coats of Rust-oleum anti-fouling paint.

    We haul the boat out in a few months to get the prop shafts in and aligned, so I'll be able tophotograph and report after washing and minute inspection.
     
  8. tomas
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    tomas Senior Member

    Congratulations.

    I was wondering approximately how much total area required the use of angle-grinding and how many total man-hours were spent on part one alone?
     
  9. RayThackeray
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    RayThackeray Senior Member

    The bottom is about 1,400 square feet, we spent over 500 man-hours prepping and painting (I think).

    Ray

     
  10. yojimbo
    Joined: Nov 2013
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    yojimbo New Member

    Abraided and Primed plate

    What led you to the decision not to use abraided and pre-primed plate for the hull?
     
  11. RayThackeray
    Joined: Apr 2011
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    RayThackeray Senior Member

    I acquired the bare hull already built. The steel is mostly of extremely high quality with mill scale intact even though the hull was over 16 years old.

     
  12. JakubT
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    JakubT Junior Member

  13. rwatson
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    rwatson Senior Member

    "cleanLASER offers laser beam cleaning technology for gentle and efficient surface treatment"

    Doesn't sound like it would be any good for mill-scale.
     
  14. JakubT
    Joined: Mar 2015
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    JakubT Junior Member

    I cant disagree, but look at the videos on their site.
     

  15. Andy Jr
    Joined: Jan 2016
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    Andy Jr Junior Member

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