Can Gelcoat And Epoxy Bond?

Here is the first of 2 video’s where I look to see if there’s any truth to the belief that polyester based materials (such as gelcoat) won’t bond to epoxy. It’s been long believed that if you were using epoxy for your fiberglass repairs, you were more or less limited to having to use paint as your topcoat.

Over the years I’ve “broken the rules” on occasion and to date have not had any issues using gelcoat as my topcoat over epoxy. I’m not saying that it’s common practice, but in certain circumstances it’s something that I have done successfully.

When considering this idea, there are a few things to keep in mind (and these are VERY important)!

* Proper ratio’s of resin to hardener are VIP. Too much or too little can result un-uncured epoxy. A bond is only as good as the surface that you’re bonding to. If it’s not fully cured you’re going to run into problems.

* Presuming the mix ratio’s are correct, having everything properly mixed is another important step. Scrape the sides of the mix cup, scrape the bottom; repeat, repeat, repeat!. You want to make sure that all of the hardener is incorporated into the resin. I will typically stir for a minimum of 1 – 2 minutes. If you really want to cover your bases, transfer the mixed epoxy to a clean cup and stir a bit more. This will ensure that all the hardener is well incorporated.

* Cure time and temp. Depending on the temps you’re working in, epoxy can take a few days to fully cure. It is advised that you allow your epoxy to cure for at least 3-4 days before moving forward.

* A natural by-product of epoxies curing process is something called amine (or amine blush). This is a film that forms on the top layer of epoxy that will inhibit the cure of polyester material as well as create bonding issues. How do you take care of this? Simple.. Warm water and a scotch-bright pad 🙂 Amine is water soluble and is easily removed. Scrub the surface with clean water, wipe the surface dry with a clean papertowel and this detail is taken care of.

* Clean, Clean, Clean 🙂 Make sure that the surface you’re bonding to is very clean. A wipe-down with acetone and / or a wax remover both before AND after sanding will remove surface contaminates.

* Lastly, the bonding surface needs to be well scuffed with sandpaper to ensure a good mechanical bond. I will generally use 60 or 80 grit paper and sand the surface until it is uniformly dull in appearance.

Something that I want to emphasize is that the above steps are not only geared to epoxy. These are good general rules when working with composites; either polyester or epoxy. Regardless of the resin you decide to use, if the mating surfaces are not fully cured and clean it may come back to haunt you!

Next video will show the results of the tests!

9 Responses to “Can Gelcoat And Epoxy Bond?”
  1. chris elkins says:

    I have heard the epoxy used is critical. Also I have heard West system, the one used in the video, is one epoxy that can be used under gel coat.

    Second issue. Those of us using epoxy never put gel coat directly on top the epoxy. We generally use a high build epoxy primer, and then a finish primer and then a 2 part polyurethane.

    We dont want to have to sand the epoxy but generally would prefer to sand the high build as its a lot easier. Just wondering why you did not test the high build????



    • ~Andy ~Andy says:

      Hey Chris,

      Sounds like the process you outlined is prep for painting? That’s pretty much the exact steps I take when spraying Awlgrip 🙂 I put this demo together for folks who are looking to use epoxy for their repairs but still finish off with gelcoat to blend their work. Epoxy has a lot of appeal for repair work (extended work times, stronger mechanical bonding, better moisture protection, etc). Most DIY owners are typically working on boats that have a gelcoat exterior, which is what the focus of this experiment was geared towards.

  2. Dominic Iadicicco says:

    Hey Andy,

    I keep hearing that a product call System Three SB-112 is an epoxy that is 100% with Polyester. They claim that you can use it as a tie Coat.

    Also what about Pettit’s Tie Coat Primer? Could that work. It claims it kills the blush. I can’t speak from experience.

    All the best.

    Happy Holidays.


  3. Groundthing says:

    I have been using west system to plug holes left in my transom from previous owners accessories. I have found if you grind down the surface of the west sytem after cure well below gel coat with a dremel grinding bit and then use 3m above and below filler to act as a bonding agent between the new gel coat and epoxy resin, the gel coat seems to set up just fine. I get so frustrated at gel coat that wont set up. Im a novice at this stuff but am a pretty handy guy, your videos have helped tramendously. Dont listen to the critics about your videos, the education of these products are money saved in my pocket.

    Thanks for the info!

  4. Alessandro says:

    Hi Andy
    Ok gelcoat over epoxy.
    Now I have a doubt.
    Use the epox to repair a polyester laminate is correct? The repair is safe?
    To Repair a laminate polyester of my boat which resin you advice? Polyester or epoxy?
    The crack is below the waterline.


    • ~Andy ~Andy says:

      You’re safe to use poly above or below the waterline. If below the waterline I’ll typically do any fairing with thickened epoxy as it provides a better moisture barrier. Once the shape is right, then a coat or two of bottom paint 🙂 Hope this helps!

  5. Pat Chouinard says:

    I am thinking about sanding down stress cracks on a Laser hull and then laying in fiberglass cloth using the 206 epoxy based on your test followed by bottom paint.

    Logically the test results should be independent of whether epoxy is applied to gel coat or gel coat is applied to epoxy. Or do you have a different recommendation?

    • ~Andy ~Andy says:

      Hi Pat! If you’re looking to prevent stress cracks from coming back, I’d sand the surface down to bare glass, then apply the new material followed by bottom paint. Its aways best to apply glass onto glass (when possible). Hope this helps! Thank you!!

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  1. […] is the first of a two-part video series clearly demonstrating the techniques Andy used for his challenge, following up with the results as […]

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