No polyester on top of epoxy?

Discussion in 'Materials' started by Canracer, Dec 11, 2014.

  1. ondarvr
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    ondarvr Senior Member

    I will normally use Atprime 2 if I need to improve the bond on difficult substrate.
     
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    If you put polyester or vinylester over epoxy, it has to be fully cured, not green or a day old, but actually cured (a week at least).
     
  3. Canracer
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    Canracer Senior Member

    george davis, I'm working on this 15 foot Coronado (there's a light bulb inside and that blanket keeps the heat in.) The last day out, she took on a little water, and project "find the tiny leak" turned into something much bigger.
    [​IMG]


    These are the forward ends of two 5' stringers that went in yesterday. They are bedded onto 6" wide strips of biax with stitched mat backing. They start at that bulkhead (halfway between the bow and the cockpit hatch) and run aft along each side of the trunk. The bulkhead tabbing looks better in real life, the grinding dust is making it look terrible. I'll cap the stringers with a layer of lighter 4" tape (6ounce.)
    [​IMG]


    I'm not building the major structure of the boat, just adding extra stiffness. I have always used WS epoxy, but I wanted some experience with polyester (also, it's about 1/4 the cost.) I'm planning to add 3 frames under the cockpit, and I want those stringers as connection points (no frame edges directly against the hull.)

    This diagram shows the frame locations. The glass I added is a bit overdone but the weight is bellow the water line and I can lighten the boat in other ways. The bonding surfaces were well keyed and cleaned. I also have a new centerboard and rudder, both with much more lift (the factory foils were an absolute joke,) and extra reinforcement will help keep the hull stiff, and hold the centerboard trunk in one piece.
    [​IMG]
     
    Last edited: Dec 14, 2014
  4. PAR
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    PAR Yacht Designer/Builder

    If you'd like to stop making the West System folks rich, use one of the secondary formulators, like Bateau.com (Marinepoxy) or our forum member's products at epoxyproducts.com. You'll pay less than 1/2 of what West System charges, for the same physical properties.

    As to using polyester or vinylester over epoxy, I wouldn't in stressed areas and the hull shell where you're applying it would be fairly heavily stressed. Marinepoxy from Bateau.com will cost you $60 a gallon, if you buy small quantities, with larger quantities dropping this down to $45 a gallon.
     
  5. Canracer
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    Canracer Senior Member

    I made some previous repairs with epoxy, but so far I've ground all that away. It's been all poly on poly (except for one small area where the stringer glass overlaps the bulkhead tabbing.)
     
  6. DennisRB
    Joined: Sep 2004
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    DennisRB Senior Member

    I heard that you could not get poly gel coat to stick to epoxy. I had to do some structural repairs on a poly boat.

    http://2.bp.blogspot.com/-Jy0jAFDB0...AwQ/NnvBQOHilbY/s1600/P1030042 %28Copy%29.JPG

    The repairs are no doubt much stronger in epoxy. The epoxy sticks to the old poly better than new poly would. The epoxy laminates are stronger anyway and wont crack as easy. I faired in epoxy too.

    The issue was I wanted to use gelcoat over the top.

    I done a few tests. I had some marine ply I was using to wet out the glass with epoxy. This had cured for only a couple of days. I put some big blobs of gelcoat on the sheet without sanding it or washing it (to make the bond as bad is it could be) for the first test.

    A couple of days later I tried to chip the blob of gel coat off thinking it would just pop off the epoxy. Well the marine plywood gave way with much difficulty and I ended up with a blob of gel coat with epoxy stuck to the back along with plywood stuck to the epoxy!

    I did not proceed to test number 2 as I was convinced it would stick well enough for a cosmetic gelcoat finish.

    The west system site explains how do it properly. Make extra sure you have no excess hardener in the epoxy (as poly will not stick to the amines). Allow as long as possible to cure, IE over a week. Wash the epoxy first, then sand it with a coarse grit before adding the gel coat. I done all that and am confident it will stick considering I done none of that it my test and the bond was stronger than the surface of marine plywood.

    I am sure the same procedure will work for applying poly laminations on epoxy too. But as mentioned it still could not be relied upon for anything other than cosmetic or light non critical structural work. Try a test like I did to see if you are happy with the result. Perhaps laminate a few layers of poly/glass on with a block of wood then try to rip the block of wood and laminations off the epoxy?

    http://www.westsystem.com/ss/polyester-over-epoxy/
     
  7. pauloman
    Joined: Jun 2010
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    pauloman Epoxy Vendor

    from personal experience and from feedback from customers - polyester resn with fiberglass cloth seems to bond to wood for a fews years before disbonding....

    paul
    progressive epoxy polymers
     
  8. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member

    Cloth is about the worst product there is to use with polyester, it has no properties that are useful in bonding to anything. This is one reason for the idea that polyesters bond so poorly to anything, even itself.

    Many people that use epoxy try to use polyester in a similar manner, and cloth being very popular in the epoxy world leads people to use it with polyester.

    One of the main reasons for poor bonds and weak laminates is directly related to cloth, the fine weave and low resin content that works so well with epoxy, is its downfall with polyester. It is so thin and flexible it can be peeled off rather easily because the entire load is being transferred to a very thin layer of resin at the bond line, which can be brittle, so it fractures and fails.

    Mat needs to be used as the first layer, and in between every layer of woven or stitched fabric in hand lamination, not doing so is an almost guaranteed way to make a laminate that will fail easier and sooner than expected.

    The popular term "boat cloth" should be banned from use, people think, gee, if it's called boat cloth it must be a good product for boats. With epoxy yes, with polyester it's definitely a big NO.

    Plus people rarely use the correct methods to get a good bond to wood when using polyester, and in the production world the correct methods are used even less often.
     
  9. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I've found the exact opposite is true, in regard to the transfer of techniques and products. My experience has shown, folks familiar with polyester, bring these techniques to epoxy efforts. They'll use mat, when it should have been cloth, they'll fool around with hardener ratios, just like they did with poly and get a gummy epoxy mess, etc.

    I agree they should remove "boat cloth" and several other advertising and marketing terms from use. The first one on the list should be "universal installation" or "no tools needed" and these terms should be replaced with "guaranteed not to fit" and "no brains required"
     
  10. ondarvr
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    ondarvr Senior Member

    Yes it does go both ways. Typically people familiar with epoxy do a better job than the polyester crowd when it comes to switching resins.
     
  11. Canracer
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    Canracer Senior Member

    Speaking of personal experience, here is a WS epoxy project. (These images are a bit large.)

    I worked out this foil pattern on a sheet of graph paper (wood fiber.)
    [​IMG]

    To attach the pattern to 1/4 inch plywood, I applied epoxy on top of the paper. In THIS circumstance, epoxy would not penetrate a sheet of paper. I got lucky and the epoxy wicked around the edges, and the project was not a total loss.
    [​IMG]

    Now, WS reports that a strong and durable bond is not necessarily dependent on deep absorption into the wood fibers. However, I was a little surprised that this epoxy didn't wet out a thin sheet of paper.
    [​IMG]
     
    Last edited: Dec 16, 2014

  12. Canracer
    Joined: Aug 2009
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    Canracer Senior Member

    It's important to note, that paper includes binders or seizing, not only wood fibers. They might be impervious to epoxy.
     
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