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Whats the difference between gelcoat and polyester resin? #736271
11/05/05 05:54 PM
11/05/05 05:54 PM
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Just curious as I can't find any info on the web about the specific differences between the two? What is the composition of gelcoat? And can pigmented laminate resin essentially be used as gelcoat?

Re: Whats the difference between gelcoat and polyester resin? [Re: socow] #736272
11/05/05 08:01 PM
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Pigmented on tinted lam resin stays tacky, so not good for final outside coats...
Poly resin is just that, period, for polyurethane blanks.
Gel coat is a pigmented resin normally associated with molding process's of construction, so is the first coat applied to a female mold. Then some reinforcements and then maybe some injected or shaped foam....two pieces, top and bottom.
When the plug is removed from the female mold, it can be one or two pieces, so need to be put together, then final clear coat finish's usually applied.
Gel coats can be either epoxy or poly resin, but it's the outside layer, except for the clear coats, that are usually sprayed first onto the mold release of the female plug.


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Re: Whats the difference between gelcoat and polyester resin? [Re: LeeD] #736273
11/05/05 08:36 PM
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Hey Leed,

As you may have guess this is a boat related topic. But I use my boat mainly for getting to surf so it should be legit. I digress.

Anyways my question was more geared towards what is the composition of typical polyester gelcoat? I know it is essentially poly resin but do they add something or change it somehow? I was told by a guy at fiberglass hawaii that its thicker than regular lam resin. What makes it thicker? Filler? If so what filler? Or is it Poly resin manufactured a different way from lam resin? What do they add to make it gelcoat instead of just regular pigmented resin?

Last edited by socow; 11/05/05 08:38 PM.
Re: Whats the difference between gelcoat and polyester resin? [Re: socow] #736274
11/05/05 08:52 PM
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I think now you are talking specific requirements that the different boat builder spec for their needs, which can be really thick or kinda thin resins.
Some builders say so and so spec, while other's do it their way.
I know sailboart gelcoat is usually the cheapest marine resin mixed with really fine glass strands for bonding, so need some finishing after they are removed from the plug.
If you boat was a higher end model, then you'd have to ask the manufacturers exactly what to use to replicate their gell coats.
Me, I fixed quite a few gel and structural holes in sailboats, distained using marine resins, and using surfboard resins, everyone thought the repairs were done professinally well.
Hot/sanding resin mixed with microballoon, very little, then usually white, or other colored pigments.


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Re: Whats the difference between gelcoat and polyester resin? [Re: socow] #736275
11/06/05 07:36 AM
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I'm not particularly sure you're getting the right information. I spent years fixing boats, rebuilding boats, fixing boards (polyester resin & epoxy) and glassing boards so I know a little.
Gelcoat is a pigmented ortho- or isophthalic resin or vinylester resin; some thixotropic agents (thickeners) can be added. Cabosil comes to mind which are little silicate hook-like things to thicken the resin, paint, etc. Gelcoat can be formulated with or without a "wax" surfacing agent depending on the use. If you spray into a mold, you would not want to use the gelcoat with wax. Wax additive rises to the surface of the laminate blocking air from the resin allowing tack-free setting. In a mold, a clear gelcoat is first sprayed, followed by colored gelcoat to about .015". Once this "gels", cloths are laminated to build to desired thickness for desired purpose. For a boat, a laminating schedule may be a very light cloth followed by a light strand mat followed by woven roving. Each woven layer is preceded by a strand layer which is why bi-ply fabrics are used. A Hot Tub may just be gelcoat in a mold followed by chopped strand mat shot at the mold from a gun.

Fixing boards is different from fixing boats although many of the techniques are the same; the materials one uses also differ slightly but may be the same. You can fix each with different end results of quality and its the experience and knowledge of the technician who knows the materials he's working with and on who will determine what the finished product looks like. I hope this is helpful.


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Re: Whats the difference between gelcoat and polyester resin? [Re: socow] #736276
11/06/05 07:40 AM
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I got a little off the subject. Your essential question was the difference between gel coat and resin. The big difference is the wax part of my answer. Each can come with or without wax. It really depends on the application. Explain the application or repair you're working on and I might be able to give a better answer. There are thousands of formulations of resins and epoxies and a thousand ways to do every type of repair.


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Re: Whats the difference between gelcoat and polyester resin? [Re: i,surfjac] #736277
11/06/05 08:22 PM
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Hi i,surfjac

I was more curious about what the typical gelcoat composition was. I see now that it varies. I have a deep V Avon Searider rib and was going to do some floor glassing as well as touch up some scratches and hairline cracks to the hull. I've heard using Epoxy for the scratches would form a much better bond than poly based resin. In any case the scratches on the outside are more cosmetic but the floor inside the hull needs to be reinfoced. There seems to be some flex in some spots especially under the coxswaines seat. We get lots of air time and jumping in the seas. Thanks.

Last edited by socow; 11/06/05 08:24 PM.
Re: Whats the difference between gelcoat and polyester resin? [Re: socow] #736278
11/06/05 10:44 PM
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Remember, gel coat is NOT the strength, but the appearance enhancer.
You need to layer glass, the thickness depending on your needs for flexibility.
Prep is one big key, well sanded, super dry, slightly rough.
What material are you reinforcing? I assume wood.


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Re: Whats the difference between gelcoat and polyester resin? [Re: i,surfjac] #736279
11/06/05 10:47 PM
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thanks i,surfjac
that cleared alot, i already knew alittle of the boat building proses.
ive repaired lots of boards, ive had people ask me to fix there jetskis, which i declined, i know the resins are differnt, im not comfortable repairing something that needs to support so much load.

Re: Whats the difference between gelcoat and polyester resin? [Re: socow] #736280
11/06/05 10:51 PM
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You know, you could contact Avon and give them the hull number. They probably could give you, sell you, gel coat from the same batch they used when they made the boat. As to the cracks in the floor, they usually form from too thick gel coat and not enough reinforcement not to mention your proclivity towards jumping the boat. I always hated fixing gel coat cracks and/or crazing. I would tell people to paint the hull or deck, its easier although usually not cheaper. There's a vinyl mat product, the name of which I forget, which works great on the deck. Like Astro-deck, sand fill, lay it on with epoxy, lasts forever.

I'm currently at the Florida Int'l Boat Show. I called my wife earlier, who also works in the marine trades, and told her I should've been a farmer. I hate boats. Well, I like my boat.


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Re: Whats the difference between gelcoat and polyester resin? [Re: LeeD] #736281
11/06/05 10:53 PM
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You can get almost perfect color matched gel coat from Spectrum gelcoats in Washington. Color matching is an art since every corner of a yacht has faded differently even more so with colors.....good luck! Gel coat is pretty crappy stuff in reality- fades ,very porous,little UV.etcMost high end boats use epoxy, and primers and linear-polyurethanes(sterling,awlgrip,imron) repair with bondo ,spray repair with gel coat in prevail can sprayer final spray with PVA(poly vinyl chloride) dry overnight and wet sand and polish.

Re: Whats the difference between gelcoat and polyester resin? [Re: i,surfjac] #736282
11/07/05 01:09 AM
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Yeah prep for sure. I take a drill with wire brush and really grind the hell out of the old grp to get some bite.

I should give Avon a call about the floor. Mines a newer ex-mil orange hull design and I've heard they no longer used wood for the floor. But I did a drill test and theres something fiberous under the glass floor but I can't figure what it is. Definitely not wood though. In any case several layers of glass mat or even surfboard glass sheets in several layers otta beef up the floor. I really don't care too much for the spider cracks on the outer hull anyways. As long as it isn't structural which it doesn't seem. These boats were made to jump!

So final Q, can you use pigmented laminate resin as a gelcoat? What would you need to add to laminating resin to make it like gelcoat? Or can you? Thanks.

Last edited by socow; 11/07/05 01:14 AM.
Re: Whats the difference between gelcoat and polyester resin? [Re: socow] #736283
11/07/05 01:18 AM
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Seems you're using gelcoat in a different context than normal boatbuilding industry.....
Normal boat building, mold release into a female plug, then gel coat, then glass, then matt, then wovenroven, as said before.....
But you're using gel coat possibly as the outer, final layer, so lam resin won't fully set.
In molded boat building, lam sets with the absence of air, as it's between mold release and glass, and the mold itself.
In your application, I suspect you are laying multiple layers of glass, then finishing it with "gelcoat" for looks and smoothness.....which you should be calling hotcoat and finish coating, and use wax to make the resin set hard.


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Re: Whats the difference between gelcoat and polyester resin? [Re: skip] #736284
11/07/05 01:49 AM
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repair with bondo ,spray repair with gel coat in prevail can sprayer final spray with PVA(poly vinyl chloride) dry overnight and wet sand and polish.


You can buy clear gelcoat and add orange pigment to match or get it from avon or Spectrum .You have to spray PVA over it to get it to kick. I think I have seen you in your boat in Channel Islands harbor??

Re: Whats the difference between gelcoat and polyester resin? [Re: skip] #736285
11/07/05 02:09 AM
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Hey Skip,

Yeah you must have. I've launched from there quite a bit. What boat were you in?

I'm not gonna bother with painting the exterior. I just wanna reinforce the inside floor. I've sprayed thinned out resin with the disposable Prevail stuff. It works nicely for small jobs.

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