Gelcoats... what are they? :-)

[ErikG]

There are numerous types of Gelcoats around does anyone have a good web page describing their technical properties or have the time and energy to explain it here I'd be very grateful!

For example:
Polyester Gelcoat, are prone to blister problems
ISO / NPG Gelcoat, claimed to be blister resistant. Are they?
Vinylester Gelcoat, used at many yards to prevent blistering
Polymer Gelcoat, what are they?
Epoxi "Gelcoat", afaik it doesn't exist but coloured epoxi with UV protection could do the job to I guess.


All kinds of information welcome
Any thoughts on gelcoats in general.

Erik

Gelcoats are basically filled (with thixotropes, pigments, extenders, UV stabilizers) polyester resins that are formulated to cure in thin films (around 16-25 mils). I believe there aren't very many commercial vinyl ester gelcoats since VE's have a tendency to have cure problems in thin films.

I believe the "blister resistance" is referring to the hydrolytic stability (ability to not breakdown in the presence of water) of the gelcoat. Run of the mill ortho gelcoats are somewhat soluble in water, when compared to higher end resins, and once the molecules breakdown and form a solution with water, a seed site for a blister is formed. ISO-NPG gelcoats have a much better hydrolytic stability and do not break down as readily.

I believe the reason that we don't see epoxy gelcoats is due to the cost (and maybe the finish) is producing them. Gelcoats are a cheap in mold coating, and it may be cheaper to just use urethane paints rather than epoxy gelcoats.

Some fairly good sites for gelcoats may be:
www.cfa-hq.org
www.ccponline.com
www.interplastic.com

[turnershells]

I've tried vinyl ester gelcoats on my hulls and found them too brittle - the finish cracked getting the part out of the mould. As a tooling resin they work fine. One of the better examples of how a more expensive material may not necessarily have superior performance.

I've used Duratec HiGloss as a thinner for both polyester and vinyl ester gelcoats - the only problem being if you thin too much you tend to get pin holes, and you have to respray later or fill the holes by hand. You still need to use a pressure pot, but it sprays with otherwise conventional equipment.

My boats spend most of their lives out of the water so blistering has never been a concern - spraying a thin and light coating is more important to keep the weight down.

Never heard of an epoxy gelcoat and it was my understanding people would very much like to find one for use in high-end aerospace composite work.

Hope that's useful.

Matt Turner
Turner Racing Shells Ltd.

Very useful - thanks Matt!

I was wondering myself about alternates to polyester gelcoats and whether vinylester gelcoats would be a good thing to try... I appreciate you sharing your first hand experience...

Epoxy gelcoats would indeed provide the blister resistance you require. However, even with UV absorber additions, epoxies still fade and chalk severely relative to urethanes. They simply lack the gkloss and color retention. You could feasibly improve blister resistance throyugh using an epoxy primer prior to application of the gelcoat. FYI: urethanes are traditionally much more expensive than epoxies.

Mike Jackson
Specialty Coating Services, Inc.
www.scslabs.com

Is there any reason that I could not use a 2 part urethane paint over the epoxy barrier coat, then the bottom paint?