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Old 29-08-2017, 22:53   #1
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Gelcoat 'hardened' but is loading up sandpaper. What now?

I helped a friend apply gelcoat to his dinghy a week or two ago. We added MEKP at 1% at about 88 degrees air temp. The gelcoat hardened enough that you couldn't put a thumbprint in it within an hour or so.

I thought this was waxed gelcoat but now I'm wondering if it really was, because I came back a week later to help sand it up, and the gelcoat, despite being firm to the touch, is just gummy enough to load up and quickly ruin any sandpaper you try to use on it.

Is there anything we can do (now that it's a week or two later) to try and make it harden better? Would spraying pva on it help at this point or is it too late? What about adding a thin layer of waxed gelcoat on top (I'd rather not add much because it's really already the ideal thickness).
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Old 29-08-2017, 23:25   #2
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Re: Gelcoat 'hardened' but is loading up sandpaper. What now?

Apply an overcoat with wax additive catalyzed at 2% and put the heat to it may be the only way out.(Heat lamp or similar)

I only use 1% if I really know the MEKP is fresh as with the Gel coat, any other unknown i would use more up to 2%

Cheers Steve
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Old 29-08-2017, 23:34   #3
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Re: Gelcoat 'hardened' but is loading up sandpaper. What now?

Sorry forgot about your PVA question: application of the PVA may also work , time is getting on but perhaps worth a shot,

The Gel coat sold is normally well marked re the Wax OR NOT

Cheers Steve
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Old 30-08-2017, 00:50   #4
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Re: Gelcoat 'hardened' but is loading up sandpaper. What now?

Agree with CaptSteve, I think your hardener was too weak. It should have been solid within a far shorter window. He is also right about the heat lamps. Good luck with it. That is a tough problem, but you may come out ok with the lamps. Make sure you keep some space in there of course!
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Old 30-08-2017, 09:24   #5
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Re: Gelcoat 'hardened' but is loading up sandpaper. What now?

Gel coat and resin harden by a thermal reaction. Catalyst creates heat.
Injection molded resin cures from heated molds ... no catalyst.

So infrared heat lamps will cure Gel coat even w out catalyst.

Heat it up and it will finish curing
It may even finish in the hot sun.
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Old 30-08-2017, 09:48   #6
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Re: Gelcoat 'hardened' but is loading up sandpaper. What now?

Gelcoat's hardening is air inhibited. The purpose off the wax in Gelcoat is to rise to surface and prevent air from getting to actual paint layer PVC coating will cut off air as well and allow for curing.
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Old 30-08-2017, 11:50   #7
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Re: Gelcoat 'hardened' but is loading up sandpaper. What now?

Quote:
Originally Posted by KadeyKrogen38 View Post
Gelcoat's hardening is air inhibited. The purpose off the wax in Gelcoat is to rise to surface and prevent air from getting to actual paint layer PVC coating will cut off air as well and allow for curing.
Gel coat is simply resin with coloring in it.
You most likely have a polyester resin. There are also epoxy resins.
PVC is poly vinyl chloride... you don't have that.
The wax that rises to the surface in resins, that have that added, are for the final coat of resin. These are called surfacing agents and should not need sanding.
If you want to add another coat of resin, then you will have to remove the wax 1st and sand the surface to create a mechanical bond for the next coat of resin.
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Old 30-08-2017, 21:42   #8
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Re: Gelcoat 'hardened' but is loading up sandpaper. What now?

Quote:
Originally Posted by redpointist View Post
I helped a friend apply gelcoat to his dinghy a week or two ago. We added MEKP at 1% at about 88 degrees air temp. The gelcoat hardened enough that you couldn't put a thumbprint in it within an hour or so.

I thought this was waxed gelcoat but now I'm wondering if it really was, because I came back a week later to help sand it up, and the gelcoat, despite being firm to the touch, is just gummy enough to load up and quickly ruin any sandpaper you try to use on it.

Is there anything we can do (now that it's a week or two later) to try and make it harden better? Would spraying pva on it help at this point or is it too late? What about adding a thin layer of waxed gelcoat on top (I'd rather not add much because it's really already the ideal thickness).



Super common. Just acetone wipe it heavily till there's no more sticky.
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