Gel Coat vs Epoxy Resin

Discussion in 'Fiberglass and Composite Boat Building' started by grady, Jun 4, 2014.

  1. grady
    Joined: Oct 2006
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    Location: Scituate, Ma

    grady Novice

    Not wanting to learn too many new skills at once (read "Old Guy") I need to make some relatively small repairs on much large parts and would like to use gel coat as a finish (only experience is with awlgrip).

    One of the repairs is to a hinged flange on a engine compartment hatch, and would like to use epoxy resin to repair the area due to its superior strenght and ability to bond to poly resin as well as other materials.

    Is there any way to finish an epoxy repair with gel coat.


    Thank You
     
  2. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    If the area isn't highly stressed, you can use gel coat over epoxy. One note is to use a slow epoxy cover coat, as the last step, before "toothing" for the polyester gel coat. It'll stick and better than you think, again so long as it's not highly loaded.

    If it is highly loaded (like a slammed hatch lid might be), bulk up the area with fabric and epoxy, so it's overly stiff and stable, to lower stresses and gel coat as you like.

    Now, some out there are bunching up their panties, because I've suggested polyester gel coat over epoxy doesn't bond well. This is true on flexible surfaces, like the flanks of a hull shell or the bottom of a powerboat, but in localized areas, where you can control flex, you'll be fine. In other words, don't make a 40 MPH powerboat from epoxy, then cover it with gel coat, as it'll bust up and crack fairly quick, but if if the engine cover needs a repair, go for it, after beefing up the area.

    Reference:

    http://epoxyworks.com/index.php/polyester-over-epoxy/
     
  3. grady
    Joined: Oct 2006
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    grady Novice

    Thanks Par, I figured you'd be the first to chime in.

    That great news by the way. and a great article to boot.

    Thanks
     
  4. FishStretcher
    Joined: Oct 2011
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    FishStretcher Junior Member

    Duratec makes a vinylester surfacing product for doing just this. I had good luck with it. I think I paid $100 a gallon.

    I haven't tried it (edit- for this!- I use lots of it on a polyester boat), but Merton's in Springfield carries a nice vinylester. It is very reactive. It gels in the bottle in sunlight! Not instantly, but you have to keep it in your basement.

    It is ~$40 a gallon. It might work for that (application) , too. I haven't tried it for just that yet.
     
    Last edited: Jun 4, 2014
  5. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    Another little 'trick' to avoid problems is to repair with fabric and epoxy as PAR suggests then abrade and brush/roller a thin coat of polyester resin. Whilst this is setting ie past the gel state you can apply the gelcoat. That way there should be no incompatible mode. As far as I am aware the problems with polyester gelcoat and epoxy stem from fast commercial moulding of parts where the gelcoat is put in mould followed by the epoxy/fabric/core etc layup. For this reason products like Tycoat exist.

    I have used this technique of a thin 'seal' of polyester quite a bit on vinylester, epoxy and polyester layups. Another important benefit is that it acts as a bettter seal to any fibres that have been exposed to the surface. Yes epoxy is better as a barrier but that thin layer of polyester is better than nothing. No problems so far...must have done about 100+ repairs using this. One recent one was on a Pata Finn which is designed to flex in the bow sections above the w/l so only a 1mm layup!.

    Do not use wax (styrene monomer) in the sealing polyester coat but do use it in the gelcoat. Same 2% ratio as the catalyst.
     
  6. grady
    Joined: Oct 2006
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    grady Novice

    Thanks guys, doesn't sound like this will be a problem. the part has no flex as in a remote unviewable location, so I'm trying to refine my gel coat experience here then move on to more noticable spots....like my transom and tuna door.

    Thanks for all the tips and advice

    Tony G.
     
  7. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    I did a repair on the hull of a boat that was an insurance write-off. As an experiment, I put gelcoat over the epoxy repairs. It's been four years and the gelcoat is still holding fine.
     
  8. grady
    Joined: Oct 2006
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    grady Novice

    All positive results....Great news
     

  9. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member

    Sometimes it works OK, sometimes not, there are many variables that play into the success.

    Like Parr said, it can work if the area isn't stressed, also some epoxies and gel coats get along better than others. The resin application helps to improve the bond (VE helps more), this is the case for any time you apply gel coat to a surface, the fillers and pigments can interfere with the bond even over polyester and the resin helps.
     
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