Gelcoat over Epoxy Failure

Discussion in 'Fiberglass and Composite Boat Building' started by Klink Sanford, Sep 13, 2022.

  1. ondarvr
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    ondarvr Senior Member

    People offering solutions or possible problems that caused it can read his linked thread, he covered just about every scenario to make it work.

    This comes down to that exact batch of epoxy not being compatible with gel coat. If I had to say why, it would probably be that the epoxy didn't cure completely, this leaves reacted ingredients in the laminate. These ingredients are incompatible with polyester and VE chemistry, so the gel coat doesn't cure when applied over the epoxy.

    This is exactly why gel coat and epoxies don't play well together when either one isn't fully cured. One or both won't cure if there is still any unreacted residue in either resin.

    This happens occasionally, and possibly more with slow hardeners because they really do need to be post cured for them to fully cross-link and reach good physical properties.

    While the laminate is 4 years old and should be thoroughly cured, it possibly isn't, so the gel coat doesn't cure.
     
    Last edited: Sep 16, 2022
    fallguy likes this.
  2. Klink Sanford
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    Klink Sanford Junior Member

    How does one post cure?
     
  3. fallguy
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    fallguy Senior Member

    Finally, some light on the subject. Thanks to ondarvr!

    Post curing is done by heating the epoxy to approximately 150 degrees for two hours in a slow ramp up and a slow ramp down process.

    My all epoxy build, I post cured the hulls. I did not post cure the cabin and despite being white, I am seeing a lot of printing. This can only mean the epoxy is still moving/changing/reacting.

    This is probably why epoxy paint is the best solution. It has to cure well and relatively quickly. And this may explain why the VE tie coat did not work. It was also probably not cured well enough.

    I've used Raka and will not anymore. I do not wish to get sued, so keeping my reasons quiet.

    I used a 40' shipping container to post cure my hulls and 6 remote temp probes with a propane blaster on one end. It was quite an adventure..

    Thanks John.
     
  4. ondarvr
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    ondarvr Senior Member

    Most manufacturers supply that info on the tech data sheet. It will have a schedule that includes various times and temperatures to reach a complete cure.

    You would think that after 4 years in the sun it would have fully cured, but there may be a time window that needs to be met with this product.

    Polyesters and VEs cure better in ambient temperatures, and may achieve even better physical properties after being post cured. But most of the cure takes place in the initial cross linking, if you wait too long the post cure isn't as beneficial.
     
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  5. Klink Sanford
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    Klink Sanford Junior Member

    Is there a "backyard" way to post cure, like putting it out in the Florida sun or putting it out in the sun covered with black visqueen?
     
  6. fallguy
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    fallguy Senior Member

    At this point, there are too many variables. If the mix was off, time gone by..

    and the sun can be done, but post curing requires a great deal of thought (yes, patting my own back here, sort of)..

    I had an unvented chamber balloon on me whilst one hull was out under a black vinyl tarp and it was probably 140F inside a small closed chamber and it found a weak spot.

    So, post curing requires an element of control which is actually fairly significant..
     
  7. ondarvr
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    ondarvr Senior Member

    The sun can do a good job, but that method is only line of sight, some areas may get more than enough UV exposure, and others zero.
     
  8. Klink Sanford
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    Klink Sanford Junior Member

    How long would you say it should be put in the sun, say the next day after the applying? 3 hours, 8 hours? I am in Florida and the sun is outrages!
    P.S.- this is for future work.
     
  9. ondarvr
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    ondarvr Senior Member

    Look at the data sheet and see what is required first, then try to duplicate that process.

    Sun will help, but if the manufacturer says you need 200F for XX amount of time you won't get there by putting a white part outside.

    Plus all the physical properties of the epoxy are recorded after it has been post cured. You won't get close to them without a post cure, especially when using a slow hardener.
     
  10. fallguy
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    fallguy Senior Member

    A9F89BA4-EB06-47C6-8364-768529637DD4.jpeg E76DBD47-4D23-4EB4-BAD8-D9CAB2EFD8B1.jpeg
    You need to follow the manufacturers guidelines, like he said.

    The problem with a black tarp in the sun are the temp can get too high and result in deformation. The temp may be too low, or the temp may vary too much.

    Typically, ramp up is a couple hours to around 150F and ramp down three hours to avoid cracking.

    But this is hard to achieve in the sun because how to cool it if too hot, etc. Solar can work, but the best way would afford some way to avoid overcooking and deforming the part. Lotsa variable here...we actually put a black tarp on the ocean container to get some solar effect and when clouds came over, we saw a two-three degree swing in the container..fortunately it was not outside our range of temps needed

    Anecdotal, but my start time probe was in a lowish spot in the hull where I figured it would take the longest for it to heat up. I was correct and we used that for the two hour start time.
     
  11. Klink Sanford
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    Klink Sanford Junior Member

    I thank both of you very much for the lessons of this thread.

    Sounds to me, just a boat owner repairing his boat in the barn, that in the future I should stick to just using polyester resin (or vinylester) and fillers/fairing.
     
  12. fallguy
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    fallguy Senior Member

    Well, there are so many factors to deciding the best resin system.

    First, what are you repairing. If you want to gelcoat something, as you know, esters are the way to avoid trouble.

    Second, is the repair structural. For structural repairs and ones you want to seal from water ingress like stringers or transoms; epoxy is hard to beat.

    Third, can you tolerate the odors. Many diy are working in attached garages. Polyester is pretty wicked stinky. I used some Duratec and was able to isolate the boat build area from the house by two rooms and no heat return. So a shop and a main garage with return air in the main garage and windows cracked in winter and the smells still got into the house. It is possible I didn't set it up until the odors happened, but stink is a factor.

    Also, you don't repair epoxy boats with esters.

    I'm sure I missed a few other deciding factors.
     
  13. aaronhl
    Joined: Aug 2012
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    aaronhl Senior Member

    If gelcoat cannot harden on your epoxy, do you think Duratec would be any different?
     
  14. Klink Sanford
    Joined: Dec 2021
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    Klink Sanford Junior Member

    I gave up after the vinylester wouldn't harden over the epoxy deck. Just laying gelcoat on the deck was the easy part and it has been nothing but trouble and time lost. The difficult parts are texturing the gelcoat non-skid areas and the gelcoat smooth taped areas, then there is the getting the gelcoat right, so it is not a dirt magnet. If the easy part was a total failure, I'll just go with non-skid paint on the deck. I have a can of Pettit EZ Decks in the color that matches the hull, I'll just go with that.

    I'll just paint the deck.
     

  15. KD8NPB
    Joined: Mar 2018
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    Location: South Carolina

    KD8NPB Senior Member

    What temperature?
    What percentage MEKP 9%?
    What percentage patch booster?
     
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