Sandblasting epoxy between layers

Discussion in 'Fiberglass and Composite Boat Building' started by DogCavalry, Aug 9, 2020.

  1. DogCavalry
    Joined: Sep 2019
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    DogCavalry Senior Member

    Hi all. I'm using the heaviest biax glass I could find on my boat, and when it cures, it has massive texture. Like a pair of corduroy trousers. So to apply the next layer I need to scuff the surface to get a good bond, but as soon as I try for more than a tiny area scuffed, I'm sanding off glass. Wire brushes have a hard time getting into the texture. I read in some industry paper that sandblasting is used. Does anyone have direct experience with this?
    20200808_183425.jpg 20200808_182444.jpg 20200808_182428.jpg
     

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  2. gonzo
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    gonzo Senior Member

    I have used a sandblaster or repairs for many years. For new laminates, the easiest way is to keep on adding before it completely cures. If you only want to remove the blush/amine layer, a scrubby pad and detergent will do it.
     
  3. BlueBell
    Joined: May 2017
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    BlueBell . . . _ _ _ . . . _ _ _

    I have only used baking soda blasting on polyester for an epoxy barrier coat application.
    It was to remove the bottom paint and gelcoat on an unfaired matt/roving/matt on ply.
    It worked great but took a long time.
     
  4. fallguy
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    fallguy Senior Member

    All you need to do between coats is take off the stitching; knock down some shine. Leave no steps or address any steps with a quick coat of thixo before glassing on another layer...

    verify no blush or greasy surface as well; those must be washed with soapy watm water

    I run 40-60 grit over it. It takes a wee bit of glass. Based on my review of your pictures; you are going a bit too far down, but fine.

    Make sure to use a bubble buster for consolidation. A squeegee never gets things as flat for me. The glass needs to look good n flat after consolidating.

    Best also if you can operate within the Silvertip chem bond window of 72 hours. I realize it isn't always possible.

    The only time I have seen delamination within my project is ultra thin coats of fairing on areas that didn't get enough key (sanding).

    Blasting is unneeded.

    Also. You can try peelply. That is more proactive. But, be warned. Do not apply peelply near gelation. It will grab the glass and lift it while you adjust the peelply. And you will be very sad the next day.
     
  5. fallguy
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    fallguy Senior Member

    Watch the video. 29 minutes to 31 for application and 40 minutes for removal...

    guy is painfully slow...

     
  6. fallguy
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    fallguy Senior Member

    Here is a shorter, better one.

     
  7. upchurchmr
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    upchurchmr Senior Member

    I wouldn't do anything that cuts glass.
    A waste of your money and time.

    Better to use thinner glass and do two layers.
    Or peel ply and vacumn.
     
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  8. DogCavalry
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    DogCavalry Senior Member

    Thanks all. I'm much heartened by your help and advice, as always.
     
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  9. Mr Efficiency
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    Mr Efficiency Senior Member

    It certainly is not wise to be sanding through the glass, to any extent, I'd be more inclined to screed the surface with lightweight epoxy bog, and sand that.
     
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  10. fallguy
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    fallguy Senior Member

    a quick light low grit sanding and your advise combine well
     
  11. DogCavalry
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    DogCavalry Senior Member

    Thanks again. The confusion comes from lack of clarity in instructional materials. Even the Gougeon Brothers book isn't clear on this. Sand between cured coats? What percentage of surface area? To get to 100%, literally half my glass would be gone. That seems bad. In hindsight, what I should have done is use poly film on the wet epoxy. That would have left a very smooth regular surface. Could have roughened that in a fraction of the time, and lost no glass.
     
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  12. fallguy
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    fallguy Senior Member

    I am not a fan of the poly film. It leaves a shiny surface whereas the peelply leaves a matte surface suitable for bonding, although I prefer sanding it a tad. You could try both to determine a personal preference.

    Much of boat building is closey held knowledge. Sort of strange thet way. I think builders just work so hard; they don't take the time to educate/video tape.
     
  13. DogCavalry
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    DogCavalry Senior Member

    As far as poly goes, I'm resigned to some sanding anyway. Just getting a flat surface to sand where I'm not cutting glass with every movement of the sander would be nice. I spent 16 hours sanding it to the state in my pics. And I obviously lost some glass. It's in my skin now.
     
  14. BlueBell
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    BlueBell . . . _ _ _ . . . _ _ _

    Don't forget the objective here.
    To clean and abraid the surface.
    Shouldn't washing, rinsing, and wire-brushing (with the fiber striations) be more than adequate?
    Oh, and chemical wash/rinse at the end.
     

  15. fallguy
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    fallguy Senior Member

    Yes. But John is just learning, so he is worried about shiny areas below the sander.

    Epoxy is very strong; some shiny spots here n there are not horrid. I use a dime for reference. If I have anything bigger than a dime; I hand sand the spot with 40-60 grit early. Later after fairing moving up in grits to 120.

    I never wire brush, but that can get low spots a key and perhaps a bit better than my sandpaper method.
     
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