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  #11  
Old 04-16-2010
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Polyester is mostly used because it cost less and is easier to sand after setting. Epoxy is superior to polyester because it sets up a little harder, but it is harder to sand. If you repairing polyester just use polyester.
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Old 04-16-2010
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Another reason most boat builders use a vinyl based resin (besides being cheaper) when building boats in a mold is that the Gel-coat which is sprayed into the female mold first is also a polyester based resin and bonds well with the vinyl based resin used to make the structural hull. If you look at the instructions on almost any can of Gel-coat it will say something like: "Not for use over epoxy repairs". Having said that I have applied Gel-coat over an epoxy based repair and it is still holding up well - knock wood. I guess a good paint makes a better UV cover for epoxy repairs.
Fiberglass has been around since the 40s or 50s when the military started experimenting with it. Nowadays the technology has advanced to such a degree that the term 'fiberglass' is pretty vague and the whole industry considers itself in the business of 'composites'.
Anyone remember the days when a car body was entirely made of metal? Composites are now widely used in automobile building as anyone with a can of Bondo knows (Bondo is not recommended for use on the water BTW).
Carbon fiber (or fibre for those of you elsewhere) was developed for military aircraft and is now used in a variety of applications. Carbon fiber essentially substitutes the glass cloth with a carbon fiber fabric encased in a high tech resin. I own a carbon fiber 'cello which is actually a pretty damn good instrument and is superior to a wooden instrument in a number of ways. Wood still holds a place in my heart though - need to get wooden 'cello re-glued.
Yes, fiberglass is no longer just 'fiberglass'.
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Old 04-17-2010
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This point was touched upon, but if you use epoxy resin and the repair area is exposed then you will need to paint or cover with gelcoat. Epoxy doesn't stand up to uv light. If you aren't concerned about a finished look you can leave a polyester resin based patch unpainted.
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Old 04-17-2010
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FSMike, no I wasn't aware of that but I did use the wrong name. It was the West Systems I read as I have it in my bookmarks and jsut checked. Good thing for me to know and thanks.

So, from what I have read, vinylester/polyester is ok and probably adequate for my small repairs but epoxy would be a lot better. Got it....thanks to all of you and I'll be ordering the epoxies needed for the repairs. I learned more than I expected to from this thread. Whooda thunk it?

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Old 04-17-2010
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I am well aware that the issue is a secondary bonding question but I specifically asked Chemists and Engineers that work in the industry and they say for repairs as long as they are done properly there really is no problem using Resin, even for major structural repairs.
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Old 04-17-2010
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Originally Posted by CalebD View Post
Another reason most boat builders use a vinyl based resin (besides being cheaper) when building boats in a mold is that the Gel-coat which is sprayed into the female mold first is also a polyester based resin and bonds well with the vinyl based resin used to make the structural hull.

Just a clarification. Most builders, perhaps in excess or 95% or more of them, have historically used polyester resin for laying up hulls not vinylester. Only a select few builders lay up with vinylester. Polyester is the resin of choice for most builders, still is.

Builders like Catalina for example use only a sprayed in layer of vinylester on top of the ISO/NPG gelcoat then the boat is layed up in polyester resin. The vinylester behind the gelcoat slows or prevents osmosis from getting into the polyester resinated substrate..

For the repair the OP is doing polyester resin is more than adequate...
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Old 04-17-2010
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I was waiting for the 500 pound gorilla to show up on this thread.
Of course I meant polyester resin everywhere I typed vinyl; gel-coat being a pigmented polyester resin - not vinyl.
Thanks MS for catching my misstatements on the subject.
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Old 04-17-2010
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I was waiting for the 500 pound gorilla to show up on this thread.
Of course I meant polyester resin everywhere I typed vinyl; gel-coat being a pigmented polyester resin - not vinyl.
Thanks MS for catching my misstatements on the subject.

I figured that's what you meant.. There are some builders though like Morris who do use Vinylester. Funny thing is it is not that much more expensive than polyester but I guess when you buy it by the tractor trailer full, it adds up..
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Old 04-19-2010
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Filling hairline cracks in the gelcoat of a 20+ yr old boat? Get some marine- tex. It's possible the glass beneath the cracks has "delaminated" due to water saturation. You'll need to do some research if it's more than surface cracks. Lowes, or whatever, probably has gelcoat repair stuff that will work. Same stuff you've used on a snow machine will work fine. Clean, dry, sanded area is good
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Old 04-19-2010
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Found this on the web:

Don Casey Tip #15
Gelcoat Crazing


If the surface of your boat looks like a cracked eggshell, the gelcoat is suffering from crazing (sometimes called alligatoring). The easiest repair method is to sand the surface heavily and roll on two coats of epoxy primer followed by two coats of two-part linear polyurethane. The epoxy fills and seals the cracks, and the polyurethane restores the color and gloss.

If you don't want to use paint, you can grind away most of the crazed gelcoat and replace it with a fresh application of color-matching gelcoat paste. What won't work is to "paint" over the crazing with new gelcoat. The gelcoat will bridge the cracks rather than filling them, and the crazing will soon return.

By the way, localized crazing (as opposed to all over the boat) is almost always due to flexing of the underlying laminate. In this case, you must stiffen the affected area before you can successfully repair the crazing.
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