Primary vs Secondary Epoxy Bond

Discussion in 'Fiberglass and Composite Boat Building' started by fallguy, Aug 25, 2018.

  1. fallguy
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    fallguy Senior Member

    has anyone quantified the difference between an epoxy bond that is wet on green vs a secondary sanded bond?

    I realize this is somewhat subject to variations in substrates and epoxies and glass, but has anyone sort of averaged it out?

    Like, for example, a wet on green bond after a 12 hour cure versus sanding and bonding at 18 or 24 hours.

    In other words, is there a time when primary bonding ever begins to diminish to the point secondary bonding is preferred? Or, for example, if you are able to sand then secondary bonding is better?

    In a more practical sense, has anyone worked in a shop where you trusted primary bonding to occur if the prior layer was no longer tacky?

    My epoxy has a 72 hour bonding window. Seems a long time to trust primary bonding and not sanding, for another exmple.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    If you are worried about, do a small sample test, and see what the bond is like, under the varying conditions, I doubt there will be too much of an issue.
     
  3. fallguy
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    fallguy Senior Member

    Yes, testing leads the way for sure.

    But I wondered if this has ever been studied or documented.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Considering that doughy epoxy putty seems to adhere to lots of surfaces with often little prep, you should be OK
     
  5. JamesG123
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    JamesG123 Senior Member

    Probably not. Its a gray area that has a lot of variable that the mfg. can't control (temp, humidity, specific mix ratio, etc. etc. etc.) and are expensive to test for and (more importantly) provide warranty and liability over. Their lawyers would have said to just test the snot out of one specific method of use and make that the only official way and if a user deviates from that, they are on their own.
     
  6. pauloman
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    pauloman Epoxy Vendor

    I can help - call and we can talk about all the issues involved

    paul oman - owner - progressive epoxy polymers inc
    603 435 7199 (new hampshire)
    epoxyproducts.com and epoxylink.com
     
  7. Glenn Tranchon
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    Glenn Tranchon Junior Member

    Quick answer, the bonding strength of most epoxies is around 2000 psi. Polyester is usually around 500 psi. A secondary epoxy bond is also just as strong as a primary bond with polyester.
     
  8. pauloman
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    pauloman Epoxy Vendor

    that is 2000 psi under ideal conditions. I have seen 50 psi on garage floors
    paul - epoxyproducts.com
     
  9. Glenn Tranchon
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    Glenn Tranchon Junior Member

    The kind of epoxy you are talking about, garage floor epoxy is very different than a marine grade epoxy. There is no marine grade epoxy (west system, Raka, cpd etc) that has a 50 psi bonding strength. Apples and bowling bowls.........
     
  10. pauloman
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    pauloman Epoxy Vendor

    you are very wrong. I have been in the epoxy business for 30 years. Not much difference between my solvent free floor epoxies and my basic no blush marine epoxy used by lots of ex-west folks. Note there are other additives in epoxy paints vs epoxy floor paints (which mostly just have pigment). Bond strength is based upon the substrate and the prep. All epoxies have similar chemical and mechanical properties including bond strength. My Basic no Blush marine epoxy also has bubble breakers in it for better bond and appearance and is also used a lot for resealing pebble decks, penny floors, table tops and the base for metallic floor coatings - Calling an epoxy marine, bar top, etc. is mostly marketing. A few chemicals mix together and form a hard finish - now the sales folks decide where to use it and give it a catchy name. Note the even my water based floor epoxy (which has a lot of flex) makes a wonderful wood primer and my kevlar reinforced epoxy paste, wet dry 700, has saved a lot of boats but one of my customers uses in his commercial machine shop to bond abrasives to grinding disks. - paul oman - progressive epoxy polymers, inc.
     
  11. Glenn Tranchon
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    Glenn Tranchon Junior Member

    Sounds like your trying to market your company more then anything else. Obviously there is similarities, epoxy is epoxy but there is a reason boat builders dont use garage epoxy from home depot to build there boats. Expose your common floor epoxy to saltwater for years at a time and that throws another wrench into the mix. Fact of the matter is there isnt a single boat builder out there buying commerical floor epoxy for there boats
     

  12. groper
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    groper Senior Member

    depends on the presence of amine blush also - which is also dependant on the environmental conditions, humidity and the type of epoxy your using. Too many varibles to say 1 way or the other fall guy - just use peel ply on your edges to be secondary bonded and be done with it - no sanding required. otherwise go for wet on wet as much as possible. Easy.
     
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