Polyester repair before bulkhead

Discussion in 'Fiberglass and Composite Boat Building' started by Boatface, Nov 25, 2020.

  1. Boatface
    Joined: Nov 2020
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    Boatface Junior Member

    Hi there
    I am currently looking to replace a couple of the bulkheads onboard a 31' motor cruiser. I started to sand down the inside of the hull in preparation for the tabbing, but realised instead of sanding what I thought was the flowcoat, it was actually pigmented resin, (I think) and now I have gone deeper than I probably should have. I'm just wanting some advice on the best way to repair this? I intended on using polyester for the tabs, however, could I repair this with poly? Any ideas how to best tackle the repair?
     

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  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Flowcoat is pigmented resin, with wax added. Are you saying you think the pigmented resin contained glass in it ? Unlikely, but not impossible.
     
  3. Boatface
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    Boatface Junior Member

    Yes that is what I'm saying, because I can see the glass textured fibres, but also the thickness seems too great for flowcoat.

    Does pigmented resin necessary have to have wax in it? Surely it could be laminating resin also..
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    laminating with pigmented resin is unusual, because of the difficulty in seeing if the glass is properly wetted out. If you are being guided by the thickness, well flowcoat can be quite thick, or it may have pooled. This is not like sanding paint films, which are much thinner.
     
  5. Boatface
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    Boatface Junior Member

    That's a fair point to say that they could have pooled, but is this likely on a vertical face? What sort of thicknesses are common for flowcoat?
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    I would depend who is slathering it on, the thickness of it, and what desire there was to hide surface roughness, but 1/16" would not be out of the ballpark in some cases. I assume this was an area visible to occupants of the boat ? In any case, take a thick chip and see if it breaks easily, if it does, unlikely to have any glass in it.
     
  7. Boatface
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    Boatface Junior Member

    Thanks for your reply.

    I will try that tomorrow. So in hindsight I may actually have been correct in sanding back the inside of the hull until I can see amber colour?
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    It would be better to be in close contact with the actual laminate, if using polyester, you would probably get away with it OK without needing to be too fastidious, but it would be a better repair to be more thorough, and especially if the boat has to handle rigorous conditions.
     
  9. kapnD
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    kapnD Senior Member

    I think I’m looking at a chop finish, from the random patterns in the unsanded area.
    Can you see any fabric pattern in the deeper sanded areas?
    I can’t make any out in your photo, so maybe you haven’t sanded deep enough yet.
    Or there isn’t any fabric...
     
  10. Boatface
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    Boatface Junior Member

    I have checked the thickness from the surface of the 'white coating' to the amber below and it is upto 3 to 4mm in places. I can't see any roving, but I don't know if this would be obvious? The white surface certainly looks quite textured.
     

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  11. Mr Efficiency
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    Mr Efficiency Senior Member

    One can see fibreglass mat there, is this a part of the boat that was not visible, hidden out of sight ?
     
  12. Boatface
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    Boatface Junior Member

    It was in a locker originally, so out of sight. However, the whole inside of the hull including the coach roof appears to have the same coating. It's very counterintuitive of me to go sanding more than a millimetre and upto 4...
     
  13. kapnD
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    kapnD Senior Member

    Do you have any idea how thick the hull is ?
    I seriously doubt you’re jeopardizing it’s integrity if you can’t even see the roving yet.
    It’s pretty obviously sprayed over with chop, which can easily be several mm thick.
    I’d keep grinding, maybe use a more aggressive disc?
    Is the new bulkhead intended to be structural?
    Is the hull flexing in that area?
    If not, I wouldn’t worry too much about it, a non-structural element doesn’t need to be totally married to the hull laminate, it’s just a matter of glueing it in place securely, so a clean, sound surface should suffice.
     
  14. Boatface
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    Boatface Junior Member

    Is it likely to be sprayed chop strand? The only reason I ask is that it was made in 1969. The underside seems to be around 8 to 10mm thick and the topsides look about 6.

    The bulkheads are roughly half way along its length and bolt to the coach roof forming the cockpit. I'm not sure if they are structural, but I intended to build back as though they are. Will polyester/vinylester adhere to this coating at all?
     

  15. ondarvr
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    ondarvr Senior Member

    Polyester is all you need for this project.
     
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