Fiberglassing foam core kayak, Polyester vs Epoxy

Discussion in 'Materials' started by Bren Leach, Jun 18, 2020.

  1. missinginaction
    Joined: Aug 2007
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    missinginaction Senior Member

    Here's an old thread regarding fairing compounds. It's from six years ago but just as applicable today. BTW, Paul Ricelli (PAR) who contributed a lot to this thread passed away in 2018.

    Does a fast drying and easy to sand filler exist that adheres to epoxy?

    Here's another.....

    looking for a fileting material in a pinch.

    And another.......

    epoxy filler+thickener?

    Have fun and wear a mask. No, not for corona, to protect your lungs from the silica and dust.

  2. sebaseba
    Joined: Apr 2020
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    sebaseba Junior Member


    Well XPS is polystyrene. It is soluble in acetone, methyl ethyl ketone (MEK) and similar. You can make a glue by dissolving XPS scraps in acetone and use that as glue or filler.

    I invite you also to test my beam strength calculator that features also XPS

    Your idea is actually similar to one of the ideas I've had at the beginning. Except your hull shape is kinda weird.

    Also I was thinking to rather make it with vertical plates as glued parts probably have a lesser strength. Not sure.

    A method is also to use polyethylene foil to roll it around.

    Main problem of XPS is that it dissolve if it gets in contact with fuels, for example floating on water.
  3. Chris Rogers
    Joined: Apr 2020
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    Chris Rogers Junior Member

    You should be fine glassing with epoxy - a 5:1 standard one should be fine. You're just building a big surf-board - so the issues will be denting and delamination - and getting a shiny surface will be lots of work. I'd suggest you break the glassing up into at least top and bottom layups - and brush the foam surface with some lightly thickened (Aerosil or Cab-o-sil) epoxy before laying the fiberglass down which will help it stick. Then wet the glass through with a brush or small paint roller. Avoid too much epoxy (drips) but make sure the glass all turns clear and you don't see air under it. Use a bubble-popper or wetout roller to press the glass in and smooth it out.

    You will also want to avoid bridging inside corners - cut the glass and lap it and inch or so around those features. See this post I did on "Slip Joints" for more on why this will help you: Slip Joints and Why You Need Them! | Explore Composites!

    If you use 10oz (ish) fiberglass cloth and double some of the high-rubbing areas you'll probably be ok. 6oz is a little light but it may work if you are careful about doubling corners. Try a test piece on some scrap and see how easy it is to dent it. More glass is better because it will resist denting but weight will become an issue.

    If you wait until the epoxy and glass is gelled but not fully cured (couple of hours maybe) you may be able to roll or squeegee on a coat of epoxy thickened with microballoons (light weight filler) that will fill the weave and give you something to sand that is not the glass itself. If you are working on a dry-ish day and don't let the epoxy fully harden before applying the coat of filler you shouldn't have blush or adhesion issues. Alternative is to use peel ply but that's a headache and might delaminate your thin layer of glass when you remove it.

    So do the bottom and let it cure then flip, smooth out the seam and do the top and lap your glass an inch or two - preferably on a seam where it adds strength. Then you can handle it very carefully and sand and fair the layer of filled epoxy and maybe use some primer to seal it before painting. Careful of applying a primer over the epoxy if it is waxy or not fully cured feeling - you can have chemical issues that keep the primer from sticking.

    Good luck! It may not last forever but if it works now it should work 15lbs heavier and less water-logg-ey. Please post some pictures as you go!
    bajansailor likes this.
  4. fallguy
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    fallguy Senior Member

    Lotsa sage advice from CR above. Hotcoating the 36 grit sanded surface with an epoxy thixo that won't sag is the best way to avoid delam. Using a 60 minute epoxy; about 45 minutes in, you need to reroll the thing then roll the glass off a tube; otherwise it will stick like hell. Develop a glassing plan that allows you to work for a day or so and glass wet on green. This means after a partial cure; keep going. So; every two hours or so you put more on..then at the end; you can weave fill, etc
    bajansailor likes this.
  5. upchurchmr
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    upchurchmr Senior Member

    You couldn't delam epoxy on XPS if you tried to. The foam is so weak it will tear outside of the glue joint.
    All this "thixo" will do is increase the weight for little or no strength addition. You guys are talking about an issue that will only happen with a "real" foam core.

  6. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    you are partially incorrect sir and I mean sir

    if you abrade the surface with 36 grit paper, the surface area increases quite a bit because the 36 grit paper shears the foam

    this increases the weak shear of xps

    How much? Maybe 25%? Is it great? No. Better than not doing it. Yes.

    It also thickens the glass and makes it less than paper thin..there is some cost in weight, but low density xps is ultralight

    I have used the technique building amas based on Dierking's methods.

    The problem with not using thixotropic mixture is worse yet. If you have voids from forming xps (you will) and you simply don't bother to fill them; you will start off with a poorly made laminate on a poor shear substrate.

    I am not a fan of xps, but it has a few uses. I would never use it as done here. But if he is going to, my advice is decent.

    I don't believe I suggested epoxy delaminating. Delamination caused by foam shear would be the thing to avoid. Bedding the glass into hot fillets and using thixo over all the formed areas will help.

    I understand the polemics here.

    This is one of those times where if you are gonna do it that way; here is what I have done that seems to work better. Not trying to advise using low density xps.
    bajansailor likes this.
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