Polyester/Core/Epoxy

Discussion in 'Fiberglass and Composite Boat Building' started by trekker, Sep 29, 2018.

  1. trekker
    Joined: Jan 2016
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    trekker Junior Member

    Can I build from a mold using gelcoat, fiberglass wetted out with poly, divinycell, fiberglass wetted out with epoxy?

    Am I missing something? Would this not solve the epoxy/poly bonding issue? Or, would the core be more likely to fail bcause of the different resins?

    I really prefer working with epoxy, but would like to use gel on the hull sides and bottom.

    Thanks.
     
  2. rwatson
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    rwatson Senior Member

    It sounds like a sensible approach. I cant validate it from things I have read or done, but it sounds totally fine.
    I will be interested to read others experience.
     
  3. Blueknarr
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    Blueknarr Senior Member

    Welcome to the forum.

    I can't think of any reason it won't work.

    Epoxy bonds to poly. Just not the other way around.

    Have you looked into gelcoats specifically formulated for use with epoxy?

    Good luck
     
  4. Doug Lord
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    Doug Lord Flight Ready

    I would think the polyester shrinkage might not work too well with epoxy especially with laminates on opposite sides of a foam core? There are gelcoats made that work well with epoxy-I think you'd be much better off sticking with the same lamianate material throughout. You can talk to a material supplier or two and get the facts.
    I've made many molds using a polyester tooling gelcoat with an epoxy laminate. The problem you run into is trying to apply gelcoat over epoxy; epoxy over polyester gel coat works fine(but repairs can be a problem). Go with a material system recommended by a knowledgeable supplier.
    FMSC - Epoxy Surface Coats https://www.freemansupply.com/products/liquid-tooling-materials/epoxy-surface-coats
     
    Last edited: Sep 29, 2018
  5. fallguy
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    fallguy Senior Member

    I would not trust the cte without testing or review. For my build, I have to postcure. This would also require the poly to have a high enought hdt ... and some testing.

    Personally, I see little upside in the cost savings for the issues, but I understand. My layup is basically 800g both sides and I am up around 150 gallons of epoxy with bag losses as high as 33% (wet bag), so I would love some better economics.

    Still, thinking this through; there are some issues.

    Like, say you want the mould surface epoxy first and perforated material. Then your secondary bond would be poly to epoxy/core and you would not have been able to do the whole part in one shot.

    So much of building is about time. If you lose a lot of time, like a day per panel for certain build methods; it would be bad.

    On the other hand, if you do a monocoque layup of the inside and there is little lost time; it is less horrible.

    At the end of the day, it would need to be carefully designed for similarities in hdt, cte, and other criticals.
     
  6. Ilan Voyager
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    Ilan Voyager Senior Member

    It's not a good idea to use in a structure different resins. Their characteristics (shrinkage, resilience, elasticity etc...) can be source of problems that you'll discover too late. What are you planning to build? What method do you want to use?
    Except high tech light composite structures (ie racers, fast boats, ultralight sail boats) made with exotic fibers, there is no advantage for using epoxy in a classical sandwich fiberglass. I would see only inconveniences for an ordinary sandwich. ; cost, and a lot of epoxy resins require post cure heating if you want performances superior to a good polyester.
    You have in the USA very good polyester resins with excellent characteristics. Look at the Reichhold catalog for example. I do not see the interest of epoxy in the majority of cases of laminates with fiberglass.
     
  7. fallguy
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    fallguy Senior Member

    I am building in an attached structure and used polyester (duratec) for a mould surface. It was then I decided using epoxy was the right choice. The stink is unbelievable from polyester. It took days to change the air enough from a smallish 8' keel mould.

    Otherwise, the point is good.

    A fellow builder has to stop production in the winter for this issue, so I mention it for the op, not polemics.
     
  8. trekker
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    trekker Junior Member

    Good stuff guys.

    Thanks.
     
  9. trekker
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    trekker Junior Member

    Ok. Another question. Can I lay the hull up with gel, and Poly or Vinyl and achieve a nice finish without
    using mat?

    Thanks.
     
  10. Blueknarr
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    Blueknarr Senior Member

    Can I lay the hull up with gel, and Poly or Vinyl and achieve a nice finish without
    using mat?

    Thanks.

    Depends on the complexities of your hull shape..
    Mat helps reduce print thru and will conform to any shape.
    Coremat is sometimes used just under gelcoat to prevent other fabrics from printing thru. It doesn't conform well to compound curves.
    4 oz twill leaves minimal texture.
     
    Last edited: Sep 30, 2018
  11. ondarvr
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    ondarvr Senior Member

    The short answer is no.
     
  12. ondarvr
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    ondarvr Senior Member

    As to the first part of the question.

    Get an epoxy compatible gel coat.
     
  13. fallguy
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    fallguy Senior Member

    What ondarvr says is accurate and wise for your wishes.

    I went away from this approach for the gelcoat finish complexities and weight of the mat n gel and the quality of modern two part paints.
     
  14. Ilan Voyager
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    Ilan Voyager Senior Member

    About the posts
    1-Yes polyester stinks horribly on my opinion. Excellent aeration mandatory. The vapors of styrene are pretty toxic. Epoxy does not stink so hard but it's not good to breathe the fumes. As we used tons of resin, we ended by using full masks with a supply of filtered and humidified clean breathable air by a special compressor. The pressure was slightly positive so you never breathe contaminated air. Perfect also for sanding. No more eye problems, headaches, sinusitis and other miseries.
    2- Mat over the gelcoat is mandatory or you'll have very bad surprises of printing of the clothes. There are several threads in the forum about this painful problem.
    3- Like fallguy I got rid of the gelcoat most of the times. Gelcoat is pretty heavy, and a functional gelcoat is at least a good 1/2 mm to most often a big mm thick; thats adds many kilos and often cracks.
    I used a light mat well saturated of resin over the mold, that helps for the details and keep out the printing. And eases the subsequent light sanding. After for protecting the polyester from the water, a barrier of epoxy that seals any defect. A primary and a 2 parts modern paint.
    In my opinion, although it seems to add more work, I found that it adds in reality little. First applying gelcoat is not a pleasure and takes time and lots of care. Second gelcoats have a tendency to crack and to change of color pretty fast, and are not excellent barriers to water and water vapor. Modern paints are more efficient and do not make hair cracks, you have also a far better color that does not fade and a nicer gloss.
     
    fallguy likes this.

  15. trekker
    Joined: Jan 2016
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    trekker Junior Member

    Can you suggest a 2 part paint that can remain in the water long term?

    Thanks.
     
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