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Old 19-03-2013, 13:53   #1
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replacing gelcoat

It appears the gelcoat on my little yacht is lifting and cracking in places and is due to be replaced.

There's a fair bit of clutter on the web regarding procedures for applying new gelcoat but no two instructions are the same.

Hope someone with experience can share a DIY method or maybe recommend a site with simple, reliable instructions.

cheers
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Old 20-03-2013, 15:46   #2
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My limited experience is that gel coat doesn't just crack and lift on its own. something is going on underneath to cause that. in my case I ground off the gel to expose the fiberglass underneath and found that each crack was over a void in the glass work under it. So ground that out and I am in the process of putting it back now.
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Old 20-03-2013, 16:57   #3
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Re: replacing gelcoat

I'd disagree that gelcoat won't crack and lift on it's own. Old gelcoat can do exactly that, just like old paint. If it were me, I'd seriously consider repairing damaged gelcoat areas and filling and fairing with epoxy fillers and then painting with a good two pack paint, preferably using a spray gun otherwise the roll and tip method.

If you really want to replace the gelcoat, you fill and fair with polyester or vinylester and then apply gelcoat (or rather flowcoat) with brush or spray (again, the latter not just preferably but perhaps mandatory). The big problem with replacing gelcoat with gelcoat is that it needs to be sanded smooth after final application as there is no way to get it perfect outside of a mould. Make no mistake that this is a big labour intensive job and the consensus from anyone that has done it would probably be that they wouldn't do it again. Having said that, I've sprayed flowcoat mixed 50% with acetone with good results that required no sanding, but it was on my swimming pool and not my boat. Had it been on a boat it would have required some rubbing back as it wasn't smooth enough for a boat finish.
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Old 20-03-2013, 17:04   #4
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Re: replacing gelcoat

I would get rid of all the peeling and loose bits, then first sand and roll a coat of epoxy on, then fair it out with epoxy + fairing filler, then sand, sand, sand and then spray a 2-part poly-urethane over it and that's it
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Old 20-03-2013, 17:11   #5
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Re: replacing gelcoat

Paint, don't re-gelcoat. It is easier and comes out looking better.
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Old 21-03-2013, 12:49   #6
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Re: replacing gelcoat

Thanks to all for the replies. And perhaps have come to my senses. I will go into it with the intention of sanding, filling and painting, and will only go the distance if forced to.

Yesterday my wife bought me a Festool Rotex 125 sander, so life is good.
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Old 21-03-2013, 12:54   #7
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so if one is going to paint anyway what is the purpose of the gel coat? what are the arguments for re gelcoating then painting versus just painting? I'm interested because I'm almost to this point and I have not bought the gel yet.
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Old 21-03-2013, 13:53   #8
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Re: replacing gelcoat

We did a little gelcoat repair a while back. I documented a bit of the process here
You can click on a photo to enlarge it if you want to see more detail.

Because our original plans to use West Marine gelcoat were thwarted because of a bad batch - we opted for Elmers Fiberglass Repair that we got at Home Depot. Basically used a dremel to cut down to a good surface, filled in the grooves with the Elmers, sanded and painted with some PPG 2 part "Concept" automotive paint.. A little time-consuming, but fairly inexpensive. It worked terrific - you can hardly tell where we made the repairs now. It's been almost 2 years now and those areas look great.
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Old 21-03-2013, 16:32   #9
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Re: replacing gelcoat

Gelcoat cracks because the laminate underneath is flexing and/or it's bent around too tight a radius. If the gel coat is applied too thickly, very little flex will cause it to crack. If the gelcoat application is proper thickness, takes more pronounced flexing to cause it to crack.

Our Westsail 32 must have been laid up on a cold day in SoCal (February). Apparently the applicator kept spraying on the gel coat to cover the vertical surfaces as it ran down oand built up on the horizontal surfaces. The cabin top and walkways turned into a mass of cracks after a couple of years of cruising. Tried reaming out the cracks and filling with thickened epoxy but the cracks kept coming. Finally had to grind off the gelcoat on those surfaces to cure the problem. Used a Sander/Polisher with max 3000 rpm and an 8" foam pad and 100 grit sand paper. Messy work but the low rpm of the polisher made it easy to control and didn't melt the surface like a high speed grinder will. The foam pad's flexible surface kept me from gouging into the laminate. Painted the decks with epoxy primer and 2 part paint. Still looks good after 15 years.

If the problem is caused by flexing of the laminate and/or tight radius joining surfaces, My painter said to just use a quality epoxy primer without grinding out the cracks. The primer is flexible and fills the small cracks and imperfections and won't reveal moderate cracks underneath. The two part polyurethane paint coating is very thin and flexible so not prone to repeating the cracking. If you want to be absolutely certain no cracks will print through, scrape or grind out the cracks and fill with epoxy. The guys who sprayed my boat used AwlGrip epoxy primer but another manufacturers LPU top coat. They said the primer is the key to a great looking, long lasting finish. Of course you have to sand the primer. A year later, none of the areas with cracks has come back
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