No polyester on top of epoxy?

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

  1. Canracer
    Joined: Aug 2009
    Posts: 621
    Likes: 9, Points: 18, Legacy Rep: 47
    Location: Florida

    Canracer Senior Member

    Just wondering. Why can you apply epoxy onto polyester, but you can't do it the other way around?

    At the end of the day, it's a layer of epoxy against a layer of polyester. Right?
     
  2. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,016
    Likes: 912, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    It isn't fair, but you'll have to live with it. The world frequently fails to conform to our expectations.
     
  3. Canracer
    Joined: Aug 2009
    Posts: 621
    Likes: 9, Points: 18, Legacy Rep: 47
    Location: Florida

    Canracer Senior Member

    What about outer space? There's no "up and down," and no "top or bottom."

    Does this rule still apply in outer space?

    (seriously, what's the deal with that?)
     
  4. pauloman
    Joined: Jun 2010
    Posts: 268
    Likes: 10, Points: 18, Legacy Rep: 151
    Location: New Hampshire

    pauloman Epoxy Vendor

    polyester resin doesn't 'stick' very well to anything but itself. That's why fiberglass boats are 100% fiberglass and not a wood/glass mix.

    paul oman
    progressive epoxy polymers inc. epoxyproducts.com
     
  5. FMS
    Joined: Jul 2011
    Posts: 611
    Likes: 22, Points: 18, Legacy Rep: 227
    Location: united states

    FMS Senior Member

    Expanding on what Paul said, when inspecting older boats built in the 70s and 80s it's common to find polyester FRP over wood bulkheads and small items including hatches, seat components, and of course transoms and stringers. The fiberglass is thick enough over transom and stringers to hold its own shape. Where a layer of polyester fiberglass has been applied over bulkheads or small items such as rear seats though, especially large flat panel areas, it's common to find some areas where the fiberglass skin is no longer attached to the bulkhead and there are areas that have an air void between the skin and wood with time.
     
  6. lewisboats
    Joined: Oct 2002
    Posts: 2,329
    Likes: 128, Points: 0, Legacy Rep: 1603
    Location: Iowa

    lewisboats Obsessed Member

    liquid epoxy will make a mechanical bond to cured Polyester resin but the poly won't make a viable mechanical bond to the epoxy. It won't even make a very strong bond to itself when applied wet on cured.
     
  7. ondarvr
    Joined: Dec 2005
    Posts: 2,636
    Likes: 389, Points: 83, Legacy Rep: 506
    Location: Monroe WA

    ondarvr Senior Member

    The two types of resin are very different chemically, and epoxy is a very good glue, it tends to bond to most substrates quit well. Polyester isn't a very good glue and isn't designed to be used in that way. While a chemist could into great detail on why, the final answer is still the same, polyester just doesn't like to bond to epoxy.
     
  8. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 482, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Polyester does stick to epoxy, though not as well as itself, it's still noty a bad bond, unless making comparisons between the two. As a rule, you'll want epoxy over polyester (as mentioned) and try to avoid polyester over epoxy, though it is often done anyway. In high stress and high vibration areas, the polyester over epoxy bond will fail quicker, because of elongation modulus differences.
     
  9. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,016
    Likes: 912, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    It is said that in the divine realm, there is no here and no there, no now and no then, no mine or thine. Makes polyester seem simple. :D
     
  10. ondarvr
    Joined: Dec 2005
    Posts: 2,636
    Likes: 389, Points: 83, Legacy Rep: 506
    Location: Monroe WA

    ondarvr Senior Member


    Kind of depends how "not a bad bond" is defined.

    I've never been able to get a bond I trusted, and I've tested it many times. It also depends on the exact epoxy and exact polyester used, there are many versions of both and some may be more compatible than others.

    So in light of the possible variables in bond strength, we as a company don't recommend it, and I personally don't recommend it either.

    There are bond enhancement products that can work very well, but they tend to be pricy and sometimes hard to find, so it's normally easier and cheaper to just stick with epoxy (pun intended).
     
  11. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,016
    Likes: 912, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Maybe improved by "keying" the surface ? I can't imagine polyester gripping to a slick epoxy surface with any reserve at all.
     
  12. ondarvr
    Joined: Dec 2005
    Posts: 2,636
    Likes: 389, Points: 83, Legacy Rep: 506
    Location: Monroe WA

    ondarvr Senior Member

    My testing was done on well cured epoxy and using 36 and 80 grit grinding discs
     
  13. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,016
    Likes: 912, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    What happens when you apply polyester to a part-cured epoxy ?
     
  14. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 482, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Yeah, that's the butt kicker and we're in the same boat, as anyone that's read my thoughts on the subject previously. I don't recommend it for anything other than small cosmetic applications, certainly no load, stress or structural considerations.

    Over green epoxy doesn't help, in fact it will usually cause a problem. It only works over well toothed epoxy and though some "tie coat" products are available, it's still not something I'd trust in loaded, structural applications, nor over large areas (though I've seen this done). Yep, these tie coats do tend to be costly, but if you're trying to apply gelcoat over large areas, possibly justifiable.
     

  15. gdavis
    Joined: Dec 2014
    Posts: 72
    Likes: 1, Points: 0, Legacy Rep: 17
    Location: belfast,maine

    gdavis Junior Member

    vinyl ester to the rescue

    I have just recently tried vinyl ester resin over green epoxy trying for a bond. No go, the vinyl ester never cured. As for scratching the epoxy with 80 or even 36 grit no go here either you still don't get any kind of what we call a burn in bond. We now use vinyl ester resin only for building hulls, decks and parts and repairs to poly ester boats. So much better than the old standby poly. It also sticks much better to wood providing it is applied correctly. I have even messed around with priming the wood with a thinner infusion resin so it penetrates the wood better. By the way, what is your application? What are you covering?................peace out
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.