Polyester over Epoxy - please help

Discussion in 'Boatbuilding' started by Manie B, Mar 24, 2008.

  1. Manie B
    Joined: Sep 2006
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    Location: Cape Town South Africa

    Manie B Senior Member

    With the ever rising costs of epoxy i would like to know if any of our members have used polyester and glass over epoxy.:confused:

    I have found that i can build / glue very well with epoxy and i can coat /paint / cover well with epoxy onto good marine grade ply wood 9mm thick = 3/8 inch.

    This is basically one very good coating of epoxy onto the plywood. If i then sand the epoxy with a 80 grit paper i get a very good surface of epoxy - not exposed wood - which i am hoping to apply polyester resin and glass mat and woven.

    This would make a big difference in final costs as the boat is now covered in 2 layers of polyester glass and then some fill coat??? and paint???:confused:

    Photos of your build or web details would be fantastic:D
     
  2. alan white
    Joined: Mar 2007
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    Location: maine

    alan white Senior Member

    Polyester sticks best to itself. Other materials, not so well. Therefore it makes no sense to bond polyester resin to cured epoxy as the bond will not be any better than polyester to wood, which is a weak bond at best.
    Epoxy layed up over roughened cured polyester will fare better. This is done frequently, in fact.
    It looks like you're bonding epoxy over wood. In that case, you would be better served laying up a 100% epoxy schedule. It costs more but its lifespan will be far longer. If you are building a modern strip type construction, you are stuck with using glass in one or two layers to build tensile strength into the hull. But if you are building with plywood, consider taping omly the seams with epoxy and cloth if economy is important.
    You'll have less abrasion resistance compared to glass sheathed, but there will be no seperation problems down the road.


    Alan
     
  3. Landlubber
    Joined: Jun 2007
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    Location: Brisbane

    Landlubber Senior Member

    Manie B,

    Generally we do not put polyester coatings over epoxy, the polyester may not set off correctly and can delaminate at the layer. Epoxy over polyester is another thing. no worries.

    Now having said that, I have used poly over epoxy, but not in critical areas, I would not recommend ever doing so.
     
  4. Manie B
    Joined: Sep 2006
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    Location: Cape Town South Africa

    Manie B Senior Member

    Thanks guys - the input really helps - as you know material costs pale in comparison to the labour.

    I want to get the boat done and not come apart.
    I love building BUT finishing is painfull.

    I wil stick to epoxy, thanks
     
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  5. JohnMarc
    Joined: Feb 2018
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    Location: Knysna South Africa

    JohnMarc <--- My ultimate goal

    Hi

    I realise how old this thread is but ......

    I am wanting to make a mould (+- 2M2) and I have epoxy gelcoat but would like to use polyester over the top of it to use chop strand .....with our lockdown here in SA I cannot pop down to buy poly gelcoat......
    it is only a single use mould so longevity of the mould is not an issue....

    Do you think I can do it and if there is any advice to special steps i should take considering the potential epoxy/poly clash I would appreciate it.

    Thanks stay safe & healthy
     
  6. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    It can sort of work, but.....

    The chemistry of the two resins are incompatible when un-cured, so the epoxy would need to be fully cured and then thoroughly sanded to get a barely adequate bond.

    This would probably allow you to get a part off of it.
     
  7. JohnMarc
    Joined: Feb 2018
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    Location: Knysna South Africa

    JohnMarc <--- My ultimate goal

    Many thanks for the response......does the epoxy gelcoat not only fully cure (flash I think they call it) when it is starved of air or is that only applicable to polyester epoxy ? I actually do have some poly gelcoat but I am worried it isn't enough..... is there a rule of thumb as to how much is needed per area squared?
     

  8. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    One gallon of gel coat will cover 80 square feet at 20 mils.

    There’s no way for me to know anything about the epoxy you have.
     
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