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Catalina/Capri 25/250 Sailor's Forums

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Gelcoat / epoxy

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Bruce Baker
Captain

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USA
402 Posts

Initially Posted - 09/22/2006 : 08:05:37

This might be a stupid question: what's the difference between gelcoat and epoxy?

Bruce Baker
Falls Church, VA
"Yee Ha" 3573
'83SR/SK

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Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 09/22/2006 : 08:54:07

Definition: (1) A quick-setting resin used in molding processes to provide an improved surface for the composite. (2) The first resin applied to the mold after the mold-release agent, which becomes an integral part of the finished laminate and is usually used to improve surface appearance. (3) High-build, chemical-resistant, thixotropic polyester coating.
A gelcoat is a material used to provide a high quality finish on the visible surface of a fibre-reinforced composite material. The most common gelcoats are based on epoxy or unsaturated polyester resin chemistry. Gelcoats are modified resins which are applied to moulds in the liquid state. They are cured to form crosslinked polymers and are subsequently backed up with composite polymer matrices, often mixtures of polyester resin and glass fibre or epoxy resin with glass and/or carbon fibres.

The manufactured component, when sufficiently cured and removed from the mould presents the gelcoated surface. This is usually pigmented to provide a coloured, glossy surface which improves the aesthetic appearance of the article.

Many marine craft, including large yachts, are manufactured using composite materials and the outer layer, or gelcoat, is typically 0.5mm - 0.8mm in thickness. Gelcoats are designed to be durable, providing for example, resistance to ultraviolet degradation and hydrolysis.

Specialised gelcoats can be used to manufacture the moulds which in turn are used to manufacture components. These require very high levels of durability to overcome the mechanical and thermal stresses encountered during the curing and demoulding processes.

Suitable resin chemistries for the manufacture of gelcoats vary, but the most commonly encountered are unsaturated polyesters or epoxies. Within each of these categories, the resin chemistries are further subdivided.

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 09/22/2006 : 09:03:34

In a nutshell, gelcoat is the thin, paint-like coating that goes over the fiberglass structure to make it look pretty and epoxy is the two part glue that is used for structural things.

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stampeder
Master Marine Consultant

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1608 Posts

Response Posted - 09/22/2006 : 09:48:39

How thick is the gelcoat on a 1980s vintage catalina?

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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 09/22/2006 : 09:51:11

One distinction to keep in mind when using either... Epoxy does not tolerate UV exposure well--it turns dark and eventually cracks. The pigments and resins used in gelcoat are intended to stand up to UV exposure. So, if you repair something with epoxy (just about the strongest and hardest resin-based material), expect any exposed resin to darken, crack, and even peel over time. Thus, epoxy coating on exterior brightwork is a bad idea unless it's thoroughly protected by something like spar varnish.

BTW, few boatmakers use epoxy in their layups--it's heavy and expensive. They use vinylester or similar resins.

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Dave Bristle
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Djibouti
10005 Posts

Response Posted - 09/22/2006 : 09:54:32

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by stampeder
 How thick is the gelcoat on a 1980s vintage catalina?
<hr height="1" noshade id="quote"></blockquote id="quote">
From the scars I've worked on, I'm guessing between 1/16" and 3/32". It can take a little fine sanding and a lot of rubbing.

Edited by - Dave Bristle on 09/22/2006 09:57:07

stampeder
Master Marine Consultant

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1608 Posts

Response Posted - 09/22/2006 : 10:32:28

I did a light sanding of my entire topsides this past spring, removing a negligible amount of gelcoat in the process.
It seemed to me that there was thicker gelcoat on the bow than other areas. I repaired minor scratches on the bottom and topsides following Don Casey's advice of etching and cleaning out the damaged areas and then using styrene to ensure a better chemical bond between the old gelcoat and the new. However, I didn't put plastic on as he recommended.
Because I did a light sanding, (removing all that oxidiztion chemicals would not remove) I had excellent color match. After polishing and buffing, the repairs are very difficult to detect.

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