Using polyester resin instead of epoxy resin

Discussion in 'Materials' started by Ittiandro, Apr 6, 2018.

  1. Ittiandro
    Joined: Sep 2016
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    Ittiandro Junior Member

    I have built a wood hydrofoil wing for my windsurfer and I want to strengthen and waterproof it. The wood ( pine) is soft and about 5/8" tapering to 1/4" at the trailing edge. I want to reinforce and waterproof it. Can I coat the wing with polyester resin on a fiberglas cloth , instead of epoxy resin?
    Polyester resin is not as strong as epoxy resin, but it costs half as much and if it can work I'll go for it.
    There is little mechanical stress on the wing when sailing at a windsurfer'd low speed, so perhaps polyester is good enough.

    What is your take?


    Thanks


    Ittiandro
     
  2. Eric Lundy
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    Eric Lundy Junior Member

    Epoxy and polyester can both be used for composite parts. Polyester could be strong enough to stabilize a pine hydrofoil, however depending on the construction, most applications are stronger with epoxy resin. Pine is a good construction material when paired with fiberglass and epoxy. Polyester could work but it's the engineering of the combined materials that will make or break the final product.
     
  3. rwatson
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    rwatson Senior Member

    Polyester Resin is NOT waterproof.
    Pine is not a great timber to get wet.

    For the few dollars on such a small project, I know what I would recommend.
     
  4. Ittiandro
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    Ittiandro Junior Member

    Can polyester be applied on a fiberclas cloth ( or even carbon cloth )for additional strength, as I would do with epoxy resin? Will it harden like epoxy resin and be sanded over?I gather it would waterproof the wood, but I can put layer of marine paint on it, too.

    Thanks
    Ittiandro
     
  5. ondarvr
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    ondarvr Senior Member

    As he already said, yes it can be used to make your part, and polyester is typically easier to use, but......

    For a small part Like this where the laminate will be thin you'll be much better off with epoxy if you want it to hold up long term.
     
  6. fallguy
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    fallguy Senior Member

    Use epoxy for that part. It’ll last longer to strength and ingress both. And the epoxy won’t stink up your workspace like polyester.

    Paint it to protect the epoxy from uv.

    How big is it? The economics are really not there if the epoxy costs you 50 and the poly costs you 25. Even if paint were considered wise waterproofing; the paint would cost you more to economy of scale. Have you priced marine epoxy paints?

    You don’t need marine paint on epoxy.
     
  7. Ittiandro
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    Ittiandro Junior Member

    How big it is? Two wings: the main wing 36" x 12", with varying thickness from 5/8" at leading edge to 3/8 " at trailing edge . The other one, the stabilizer, 22" x 7, but much thinner, approx 3/8" . I don't know if one 850 ml can of epoxy is enough for the job. If so, I wouldn't mind spending $ 30 CDN for the epoxy , but if I have to use a 2nd can, it can get kind of expensive ( relatively speaking) for a project I don't even know if it will work. Actually, the resin would cost me more than the material.. May be if I break it, I can repair it with epoxy, this time ( if epoxy can go over polyester, which I don't know)

    Thanks

    Ittiandro
     
  8. fallguy
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    fallguy Senior Member

    Figure sqft of surface of glass.

    Say it is 1 yard.

    Then multiply by fabric weight.

    Say 6 oz/ yard. At 110%, You need 6.6 oz epoxy, plus losses t0 rollers. I use 4 oz loss for a 5mm nap 9” roller. For this, I cut rollers to 3” or 4/3rd oz waste, or you can squeegee for less loss.

    The epoxy would be 8 oz or 7 squegee. I always round up.

    I prefer roller, but sometimes they lift fabric on certain stuff or at end when you don’t want. For you, squeegee.

    I realize you are metric guy, but you can do same easily in converting.

    Do it over a clean trash bag taped on edges and you can use drippings on part.

    After it greens up, do a second coat. I figure weave fill at about 2 oz per yard and usually do two coats. I doprefer a roller for weave filling; so you add that loss in again.

    For my full example, you’d need 4 oz roller losses and 7 oz fabric and 4 oz fills or 15 oz.

    Epoxy over poly is not a great idea, but probably workable, but not really wise economy again.
     
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  9. fallguy
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    fallguy Senior Member

    An 8 0z hardener and 16 oz resin would be enough for my example for 2-1 formulas

    Cost of ark at Noah’s is 14 for h and 16 for r or 30 bucks. Shipping will cost ?15-20. Like I said, about 50, but if you buy epoxy locally; less.
     
  10. Steve W
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    Steve W Senior Member

    With either resin you will want to paint it, polyester is not as waterproof as epoxy for sure but people need to keep in mind what you are using it for. This is not something you are going to be leaving in the water on a mooring, its something that will be immersed for hours at a time,not days /weeks/months so polyester is plenty waterproof enough. So if you use polyester you will paint it for cosmetics, nothing more. Epoxy is a better resin from an adhesion to the wood standpoint and it is better at protecting the wood from moisture if it were to be immersed for months. Since, as you mention, you are talking about fiberglass cloth you are really not talking structural glass so either resin will work. If you choose epoxy, unlike polyester it will be mandatory to paint it to protect it from UV. I can imagine this thing laying around exposed to UV at the beach for a lot more hours than in the water. Epoxy is very poor at resisting degradation from UV, polyester is much better in this regard. That said, if it were me I would for sure be using epoxy and painting it but I'm coming from a different place of being able to choose either epoxy or VE from inventory( I don't really use PE anymore) If you are coming from a place of having neither or a paint system and having to buy everything it will be a lot more expensive to use epoxy but in the scheme of things its not that much and I would still choose epoxy when wood is involved.
     
  11. gonzo
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    gonzo Senior Member

    Polyesters also have poor UV resistance, unless you add pigments like in gelcoat. Either one can be painted with alkyd enamel.
     
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  12. Ittiandro
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    Ittiandro Junior Member


    Thanks for your instructions. You have been of great help. I thought I would need much more resin!
    One more detail: for the 2nd coat do need to let the resin cure first and then lay an other layer of fiberglas cloth or can I do it right on the top of the fresh resin without an extra cloth?

    Thanks

    Ittiandro
     
  13. Steve W
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    Steve W Senior Member

    Much better than epoxies though, I've seen many fiberglass hatches on old boats where they left the middle without pigmented gelcoat to let light in and they lasted decades. Epoxy degrades in weeks. Again we always have to keep in mind how something will be used.
     
  14. fallguy
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    fallguy Senior Member

    Fill coats are done to fill the weave. Once the first layer of epoxy tacks up enough so a paint roller won’t stick; you can do a fill coat of epoxy only. Please note, I was not telling you to use 6 oz glass. It was for the example. The fill coats will help you get the part smoother.

    I don’t know what glass you need, nor do I know the measurements of the parts in square yards. A 10 oz glass would require about 11 ozs epoxy per yard.

    I think the 6 oz might be a little light for a hydrofoil, but 17 oz biax seems excessive. The vertical section can’t flex; so test your wood first and see if you need strength added via the glass.

    Maybe some more experienced guys can help you with the glass plan. Carbon under glass would be really strong I think; but not sure if you need that. Maybe post another question on glass schedule for windsurfer hydrofoils.

    If your parts do not flex; the glass is not doing any work but keeping them from cracking; so light glass would be okay.
     

  15. rxcomposite
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    rxcomposite Senior Member

    Polyester and glass does not adhere well to wood. We used to staple it to get some mechanical bond. Wood is a structural material, there is no need to reinforce it with glass. Glass skin and foam core works best.
     
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