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Thread: Gel coat


  1. #1

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    Gel coat

    Which Gelcoat are you guys and girls using when your airframe has a gelcoat coating on it. As far as I know Gelcoat is not compatible with Epoxy. I could be wrong but I do know that polyester resin is not compatible with epoxy. Don't ask how I know I usually use Bob Smiths and West system epoxy if this helps.
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    Quote Originally Posted by Justflying1 View Post
    Which Gelcoat are you guys and girls using when your airframe has a gelcoat coating on it. As far as I know Gelcoat is not compatible with Epoxy. I could be wrong but I do know that polyester resin is not compatible with epoxy. Don't ask how I know I usually use Bob Smiths and West system epoxy if this helps.
    As you seem to realize, "gelcoat" is a polyester product. It is usually sprayed in the mold first then coated with fiberglass and polyester resin. It can be tinted to any color desired, and is intended to be a hard outer shell for the finished part. It is usually only used on large molded parts (like full size boats, water slides, etc) where weight is not really a factor.
    Most fiberglass model airplane parts are molded using epoxy resin, so polysester gelcoat is not (and cannot be) used. There are some epoxy based "tooling gels" available, but they are mainly used for making the molds, not in production parts.
    The outer coating you are seeing on your finished parts is most likely paint or primer that has been sprayed into the mold, allowed to cure, then the part is laid up directly on the paint/primer. When the fiberglass is cured the part is removed from the mold with a perfect outer finish (or as perfect as the finish of the mold surface).
    Kevin Whitlow

  3. #3
    Moderator j.duncker's Avatar
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    If you were making something with epoxy and using a female mold you would start with a layer of plain epoxy gelcoat just as if it were a polyester resin.

    There is a downside to epoxy gelcoats it is very hard to prevent tiny bubbles which appear as pinholes in the finished article. Ask me how I know this.
    The dreamers of the day are dangerous men, for they may act their dreams with open eyes, to make it possible.

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    dbsonic's Avatar
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    I think Prestec can be used over epoxy resin(they'll shoot it over anything). But anymore I think most everyone uses automotive urethanes. Gelcoats polish out really nice though.
    P-40 Brotherhood #112

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    Thank you everyone for the information.
    It looked to me last time I did a repair on my Boomerang Elan it did have a gel coat of some kind. I'm no expert and could have mistaken this for something else. Repair came out perfect and only asking as it was mentioned to me by a fellow air plane flyer.
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    Moderator j.duncker's Avatar
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    If you need to repair either polyester or epoxy an epoxy based product sticks better. Although plain old bondo or even lightweight spackle works OK in non structural filling/smoothing.

    But I usually use West system 105 resin 206 hardener and 410 microlight filler. I generally use syringes to accurately measure the resin and hardener and mix these before adding the filler.
    The dreamers of the day are dangerous men, for they may act their dreams with open eyes, to make it possible.


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