Paint over gelcoat vs. paint only

Discussion in 'Materials' started by andy47, Sep 11, 2017.

  1. andy47
    Joined: Oct 2016
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    andy47 Junior Member

    I am building a 2-meter autonomous boat that will attempt to cross the Atlantic, so it will stay in the ocean for months. I want to paint it with a special antifouling paint. After each attempt (if I recover the boat), I would sand the old paint down, maybe repair it and then repaint it. I will build the hull from fiberglass, Kevlar and carbon fiber using the resin infusion technique.

    My first question is, can I do the resin infusion without applying a gelcoat over the release agent (wax)? I ask this, because I found an old post on boatdesign forum which confuses me:

    "For fiberglass boats, gelcoat must be used as it is the mold release agent, along with a good waxing of the mold itself, so might as well color it and pop out the finished product and save a step." (link)

    What would you recommend (1 or 2):

    1) Apply a gelcoat (the standard way - directly in the mold over the release agent), then sand the gelcoat on the finished part for better paint adhesion, apply a primer and then paint it with the antifouling paint. The primer I will use is recommended by the manufacturer of the antifouling paint (it's not epoxy primer). The gelcoat is white, the primer is gray and the antifouling paint is white.

    2) Don't use a gelcoat at all. Lay up the fabric directly over the release agent and then paint the finished part with the primer and the antifouling paint.

    Does the gelcoat under the paint+primer layers provide any advantages or disadvantages?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Gelcoat offers a barrier to water ingress, albeit a not completely effective one. It also provides a degree of abrasion resistance, and also acts to hide the pattern of fibre reinforcements from showing through. You could go either way with your project.
     
  3. Blueknarr
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    Blueknarr Junior Member

    Advantages of using Gelcoat:
    1 adhesion sanding for primer will be less likely to expose lamination fabric.
    2 Freeboard won't need separate painting.
    3 A thick gelcoat layer could be sander and polished if the mold was not ablolutely fair.
     
  4. ondarvr
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    ondarvr Senior Member

    It can be done either way.

    Places that don't use gel coat typically do it that way so they can see into the laminate and make sure it infused correctly. This makes priming and painting more difficult though, there will be millions of tiny defects in the surface after sanding that can be difficult to cover with primer. Using gel coat eliminates this problem.

    If you don't use an epoxy barrier coat you should use a better resin and gel coat. Infusion allows more glass to be near the surface of the laminate, this creates a higher potential for blistering.
     
  5. Blueknarr
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    Blueknarr Junior Member

    Clear gelcoat would will allow infusion to be inspected.
     
  6. leaky
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    leaky Senior Member

    You guys are experts here so maybe there is a problem with this idea, but if I was building a part in a mold that was going to be painted after the fact and/or if the part was to be fiberglassed in placed after, I'd be using PVA and not wax for the mold release, because PVA rinses off with water so eliminates most of the second guessing when prepping for bonding later.

    Jon
     
  7. gonzo
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    gonzo Senior Member

    Gelcoat falls under the general definition of paint. Unlike solvent based paints, it can be laid really thick. The advantage is mainly that minor scratches can be sanded and polished off.
     
  8. Blueknarr
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    Blueknarr Junior Member

    Wax is easily removed with solvent, so it doesn't pose a problem to post mold adhesion.
    For best adhesion wash away release agent then abraid, very coarse if more resenated cloth to be used, slightly less coarse if only to be painted.
     
  9. andy47
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    andy47 Junior Member

    Thank you for your valuable comments! I will use the gelcoat.
     

  10. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member

    Yes, clear works well, I forgot to add that.
     
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