A question about the expansion and fairing differences between poly and epoxy

Discussion in 'Materials' started by Skua, Sep 12, 2020.

  1. Skua
    Joined: Apr 2013
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    Location: Hunt's Pier WW NJ

    Skua Senior Member

    I recored my transom with MDO and DF plywood, and reskinned the interior with glass and epoxy. My question pertains to all of the bolt holes and thruhulls, that now require filling and refinishing, some of which will no longer be used. The outer skin is typical 30 year old gelcoat. The boat will be painted when done. I will be using some sort of epoxy bog and glass to rework the holes. Will the difference in the expansion rates of poly and epoxy be a problem in getting a good aesthetic? Here to fore my experience is with metal body work, which can give less than great results(without careful attention to materials) with the surface looking like a relief map. Any instruction or advise is appreciated.
     
  2. missinginaction
    Joined: Aug 2007
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    Location: New York

    missinginaction Senior Member

    So you removed a deteriorated core, replaced it with MDO and douglas, right? I'm hoping that you laid up something bulky like a layer of biaxle mat and slurried in a thickened mix of slow curing epoxy. Typically people then place a number of wood screws through the outer skin and use the screws to draw the core up tight against the outer skin/ biaxle mat/epoxy to get a decent bond between the outer laminate and the new core. Once the assembly has cured and the screws aren't needed any longer I just pop them out and fill the holes with a little thickened resin and then sand down before paining the transom.

    I've never had any problems with different expansion rates for poly versus epoxy. I wouldn't worry about it. I know what you mean abouut using some epoxies for auto body work. I believe that with sheet metal and epoxy as a filler there are vastly different expansion rates. With what you are doing you have a much thicker assembly than a sheet metal car body and the entire assembly tends to expand and contract as one. Keep in mind that resin, either poly or epoxy is basically a plastic and tends to be more forgiving than sheet metal. I'm not a metals expert or a chemist so remember that's just my take as a person who's used these products for years.

    What I WOULD worry about is keeping water out of your new core. Pay close attention to detail when you're filling those old unused openings. Any new openings you make should have three coats of resin applied to the inside of the hole sealing the raw wood and a generous application of bedding material when the fitting is installed. Get water in there and you'll be dealing with the same problem that lead you to replace the core today.

    For future reference you might want to consider something like COOSA Composite for the part of any project that is below the waterline. COOSA is a little pricey but it's impervious to water.

    Good luck,

    MIA
     
  3. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Probably not from an "aesthetic" point of view.
    I'd be more concerned (not worried) about structure failure.
    Are you painting it black?
    Will it be exposed to direct sun?
    Half in the water, half out?
    Pictures?
     
  4. Skua
    Joined: Apr 2013
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    Location: Hunt's Pier WW NJ

    Skua Senior Member

    All the holes will be overbored and filled with epoxy bog then redrilled, if not permanently filled. I was concerned, a bit, of the possibility that the filled ares may tend to stand proud, or drop low, of the surrounding poly area, when in the sun.
     

  5. missinginaction
    Joined: Aug 2007
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    Location: New York

    missinginaction Senior Member

    That's the way to do it. Drill....fill...drill. I'd suggest that after the first overbore is done, you coat the raw wood (around the inside of the hole) that you've exposed with three coats of unthickened resin. Then tape the inside of the opening and fill it with your thickened mix. As for the fairness of the finished product, well that depends on how good your eyes are and how fussy you are. Based on your original post you probably already know this but a good paint job is 90% prep work.

    There are a number of ways to make an easily sandable filler. You might want to do a search on this. Use as little silica/cabosil as you can and rely on Q-cells and even talc. Yep, just good old baby powder.

    Good luck,

    MIA
     
    Last edited: Sep 14, 2020
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