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Old 19-02-2019, 07:47   #1
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Mixing Gelcoat into Epoxy

I have a small bow dent to fix, but I'm out of gelcoat hardener. I tried mixing in some West System hardener and that didn't set. I was wondering if I just made up some epoxy resin and hardener and then added some of the gelcoat polyester for some color if that would work. It doesn't need to be all pretty, just structurally tight and waterproof without being super ugly.
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Old 19-02-2019, 08:17   #2
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Re: Mixing Gelcoat into Epoxy

It works well to tint it but still unstable in UV. Best would be to fill and get your hands on a can of dolphinite gel coat in a can. Not sure how long it lasts in the Dan but you may be able to keep it around for a few repairs??
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Old 19-02-2019, 08:29   #3
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Re: Mixing Gelcoat into Epoxy

No, they are two different things. If you have gelcoat, use it alone with the proper amount of MEKP (exactly the same thing as used for polyester resin), available in small tubes from most good hardware and boat stores.

You can get pigments to mix with epoxy, but you still have to worry about UV degradation...
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Old 19-02-2019, 08:37   #4
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Re: Mixing Gelcoat into Epoxy

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Originally Posted by jimbunyard View Post
No, they are two different things. If you have gelcoat, use it alone with the proper amount of MEKP (exactly the same thing as used for polyester resin), available in small tubes from most good hardware and boat stores.

You can get pigments to mix with epoxy, but you still have to worry about UV degradation...
I had also meant pigment not pre-mixed gel coat
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Old 20-02-2019, 13:32   #5
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Re: Mixing Gelcoat into Epoxy

It never got hard after 24 hours and wiped off with acetone.

So an out of the way special trip was made to get some gel coat hardener
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Old 20-02-2019, 15:44   #6
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Re: Mixing Gelcoat into Epoxy

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It never got hard after 24 hours and wiped off with acetone.

So an out of the way special trip was made to get some gel coat hardener



Well, I'm not surprised. That would be like using turpentine in a water-based paint!!.
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Old 20-02-2019, 15:51   #7
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Re: Mixing Gelcoat into Epoxy

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Well, I'm not surprised. That would be like using turpentine in a water-based paint!!.
If you say so

To me it was like a chemical hardener for a chemical hardener
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Old 20-02-2019, 17:06   #8
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Re: Mixing Gelcoat into Epoxy

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If you say so

To me it was like a chemical hardener for a chemical hardener
I'm not a chemist so I really don't understand the chemical reaction. But an industrial chemist said you must have the right number of hardener molecules to the epoxy molecules otherwise the mix would suffer the same way as a get-to-gether where there are too many males or too many females.

But this is where I get confused: I don't think it works that way with polyurethanes as the more hardener the quicker the resin "goes off".

Maybe this will help?

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Old 20-02-2019, 17:52   #9
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Re: Mixing Gelcoat into Epoxy

You need MEKP to harden polyesther or vinylester resin. Benzyl peroxide should work too Got Zit cream? No idea what ratio.
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Old 20-02-2019, 19:02   #10
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Re: Mixing Gelcoat into Epoxy

Hardener and catalyst are industry-standard misnomers. That said I still call them that too as that is what people are used to hearing.


MEKP 'catalyst' is an activator. Polyester resin is already 'going off' in the can, the MEKP brings the speed of this into real time. Usually mixed in between 1/2% to 2% addition to the batch of polyester resin. A good resin vendor supplies a mix chart.


I hear it's the same for vinylesters. I have no hands-on with vinylesters. When should a person use vinylesters?


Epoxy part B 'hardener' is a polyfunctional curative agent. Its volume or weight needs to be matched to the epoxides in part A that are being cured so that everything crosslinks properly. Running 'rich' or 'lean' does not make for the best results. Mix charts are also helpful here too, especially once you start using weight instead of volume.


Up to a point, 1-3-5% pigments and tints may also crosslink into the polymer but after that they can become plasticizers that soften the works. I have also noted that pigmented resins go off a little faster that plain ones.
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Old 20-02-2019, 19:13   #11
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Re: Mixing Gelcoat into Epoxy

Any peroxide should work as a catalyst if I recall but the 60s/70s were a bit fuzzy. I inhaled. If you need to catalyze gelcoat and have no hardener try hydrogen peroxide. Make sure you have ventilation.
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Old 20-02-2019, 19:17   #12
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Re: Mixing Gelcoat into Epoxy

Use Vinyl for all my repairs. It bonds chemically to polyester and is waterproof and chemical resistant.


From LBI Fiberglass


More than 95% of all boats, truck & car bodies made of fiberglass are made primarily of E-glass, an orthothalic polyester resin and isothalic polyester gelcoat.
Therefore, items constructed with these materials should be repaired with the same or compatible material. LBI’s 301 orthothalic polyester resin is a good choice for most repairs. It provides a chemical bond to the existing laminate with similar strength & flexural characteristics.
Ortho polyester, isothalic polyester and vinylester resins are compatible, they all have styrene (commonly known as the “fiberglass smell”) and chemically bond to one another.
When higher strength, bond and water resistance is required (such as keel, rudder repair, or out-board transom replacement) use LBI’s 302 Isothalic Polyester Resin.
To achieve the highest bond strength and water resistance use LBI’s 901 Vinylester Resin.
Vinylester is essentially a styrene modified epoxy resin. It adds excellent strength, rigidity, adhesion, water and chemical resistance.
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Old 20-02-2019, 19:52   #13
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Re: Mixing Gelcoat into Epoxy

Thanks for the vinylester info Scubaseas.


I am not sure home remedy Hydrogen Peroxide at 3% will replace commercial MEKP at 60%


Sailorboy1, back to the original title of the thread...one could try mixing 5% uncatalyzed gelcoat into some properly mixed epoxy and see if the scavenged pigments have enough strength to color the mixture to your liking.
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Old 21-02-2019, 03:59   #14
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Re: Mixing Gelcoat into Epoxy

Or use pigment for epoxy but then epoxy as gel coat has very little UV protection.



Hardener/catalyst to resin is I believe 1.5%-2% with MEKP. So 20 times that if using hydrogen peroxide? No idea if it'll work. Might add a lot of water to the mix. I have used benzyl peroxide (Zit cream) for a spot repair and it did work. No idea relative amounts of peroxide MEKP versus Benzyl peroxide.
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Old 22-02-2019, 10:07   #15
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Re: Mixing Gelcoat into Epoxy

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It never got hard after 24 hours and wiped off with acetone.

So an out of the way special trip was made to get some gel coat hardener
I realize that what I typed above isn't clear.

The gelcoat with the epoxy hardener in it never got hard. I never acturally tried adding gelcoat to a regular epoxy mix. I believe that would have probably hardened.
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