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green03

Gelcoat - what am I doing wrong?

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Applying gelcoat to some scratches, I run into the following: the gelcoat left in the pot (bottom and sides) cures nicely, gelcoat applied to the side of the hull does not, unless covered.

The gelcoat is the type premixed with wax.

Any suggestions?

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Just now, green03 said:

Applying gelcoat to some scratches, I run into the following: the gelcoat left in the pot (bottom and sides) cures nicely, gelcoat applied to the side of the hull does not, unless covered.

The gelcoat is the type premixed with wax.

Any suggestions?

spray PVA over it. You can use a Preval sprayer for small amounts

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It may have to come to that - I have some, but not currently at hand where the boat is - but why would the mixed-in wax fail to work if on the hull, but not if in the pot?

Good suggestion on the sprayer

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49 minutes ago, green03 said:

It may have to come to that - I have some, but not currently at hand where the boat is - but why would the mixed-in wax fail to work if on the hull, but not if in the pot?

Good suggestion on the sprayer

FIIK, but I've had the same issue in the past.  Too cool for it to cure without internal heat? I dunno.

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thickness. gel needs a minimum of 20 mils to cure, even with wax

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Air temp should have been fine by the late afternoon, though could have been warmer. However, the hull may have remained colder.  Good to know about the 2o mils - I may be under in some spots.

I take it the uncured gelcoat needs to come off completely before trying again?

 

 

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That's the one good thing about it - uncured gel will wash off with acetone.

I'd try sealing it from air first though.

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Well the mixed wax needs to float up to the surface to seal it from the air. I honestly don't know how well it does that on a vertical surface. Don't think the gelcoat cares how thick it is. Lots of it is applied in a thinner layer.

Cover with some plastic film and wait a day.

I'm not a big gelcoat user preferring epoxy and paint for most repairs.

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I'm normally on the epoxy / paint plan as well, this one seemed like reasonable departure. I've managed earlier repairs at the same location with reasonable success.

I thought about vertical (actually overhanging in part) surface. Seems to not have mattered for the leftovers in the cup I used for mixing, as it cured all the way up the walls.

Covered up for now, will give it overnight and reassess in the morning.

 

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8 hours ago, Zonker said:

Well the mixed wax needs to float up to the surface to seal it from the air. I honestly don't know how well it does that on a vertical surface. Don't think the gelcoat cares how thick it is. Lots of it is applied in a thinner layer.

Cover with some plastic film and wait a day.

I'm not a big gelcoat user preferring epoxy and paint for most repairs.

A big part of the cure cycle is exothermic.  You dont get that without thickness.... case in point the stuff in the bucket went off fine, not the stuff on the job.

 Covering it with plastic film is just going to make a mess of the surface and do nothing for the cure as the cycle is pretty much done. Maybe it will help it dry, it wont help it cure.

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Do it again.  Spray it with unscented aerosol cheapest hair spray instead of pva.  Pva via preval is useful in certain situations only, and only if you do it all the time, otherwise it's often  cumbersome. You have to hit it at just the right time too so it doesn't introduce more texture.  I keep a 1.0mm tipped hvlp touch up gun full, that works good. 

Re gelcoat:  If it's too thin, not enough wax to cure will rise up.  Hair spray, then a quick pass with a heat gun on low will work.  Catalyse near the top of the range for your temperature.

Mylar film is what you and not plastic wrap.  Only good on flat surfaces and small fills.  Will help cure fully because it seals the surface like a mould would.  Think screw holes or that size of repair only. Cut small squares, tape one edge fold it back on the tape(do not crease Mylar) then flip over. Handy because it marks repair sites too. 

 

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Had an acquaintance ring me one day, asking if I had any MEK.  I asked why he needed MEK.  He replied that he was doing a gel coat repair. Again, I asked why he needed MEK. It turns out that he needed MEKP.  One little letter that turns it from a solvent to a catalyst.  

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I've seen people wipe things down with MEKP - same band, different tune.

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Wonder if the very underused large plotter at work has any mylar rolls still in it... Good tip jg!

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I much prefer epoxy followed by epoxy appliance paint, because it's easy. But, I thought you needed gel coat on surfaced you were going to buff  and Finesse to a high sheen.

Is there a way to achieve that with paint? You can tell me now, even though I spent yesterday spraying gel coat.

And how does the hairspray trick work? All of my gelcoat kicked off fine, but it is always good to have a plan B.

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For the OP, if the big batch cured, you probably just lack thickness in the application, as previously mentioned. Not enough wax and maybe not enough heat. Coat with a curing agent of you choice and wait. If it's still tacky some gentle heat may help. After that I'd probably wipe it all up with acetone a respray. Caveat emptor, I'm an amateur and a lot of these posters know way more.

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With pre - waxed gellcoat always remember to stir it up before dispensing some off .  This will avoid getting inconsistent amounts of wax in the material to be used .          I much prefer to add my own wax to each batch , usually around 2% .Just  find it  limits the amount of variables that can go wrong .

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Like JSB said

wipe the area with some methyl ethyl keytone peroxide and it should cure. There should be no reason to remove it if it is still tacky. Go over it with more gelcoat; the wax hasn’t risen out of the gelcoat. In cooler weather you can thin the gelcoat with about 10% acetone or xylene to help the gelcoat get through the preval and put in a little more mekp than usual-about 3% by volume. And that will help it stay warm and cure fast.

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On 7/16/2018 at 6:18 PM, Sail4beer said:

Like JSB said

wipe the area with some methyl ethyl keytone peroxide and it should cure. There should be no reason to remove it if it is still tacky. Go over it with more gelcoat; the wax hasn’t risen out of the gelcoat. In cooler weather you can thin the gelcoat with about 10% acetone or xylene to help the gelcoat get through the preval and put in a little more mekp than usual-about 3% by volume. And that will help it stay warm and cure fast.

It depends on how long it's been, I tried this once and then it cured to a nice hard finish on the surface.  It chipped off easily though, didn't cure all the way through.  Wipe it off, start over is better IMO.  If it's within a few hours, then I'd fog it with hair spray and warm it up a little with a heat gun(I put one finger next to the repair, if your finger is uncomfortable, so is the gelcoat...).  Keep warming it and letting it cool every 5 minutes for about 20 and then leave it be.  Should cure rock hard. 

Thinner preference for me: thin with styrene first(should end up feeling about like coffee cream when you drip it off a stir stick) then thin a little more with acetone.  I find the acetone flashes off on the way out the spray tip, nice for getting good spray pattern, but doesn't let it flow on the surface easily, the styrene hangs around longer and does a better job of that.  Waxed gelcoat in thin layers has rarely worked well for me, easier to apply neat then fog with cheap hair spray over top(5 minutes or more after application not right away) or mylar as above and no problems, even the surface layer dries hard and sandable.  I've tried thinning with xylene(and mek once for giggles).  Styrene+ a touch of acetone always worked the most consistently for me.  MEK thinned gel behaved strangely in the cup after, xylene as well. 

 

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Cover the area with waxed baking paper leave a day, buy your own roll don't borrow from the missus, just trust me on that.......

If you are filling chips or voids fill with gel then put the waxed  baking paper over the spot and then drag a razor blade over it - not too many times 1-3 max this will help fill and fair before the stuff kicks off.

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It really doesn’t matter. Gelcoat with wax I nit will weather differently than your boat and in a few months your repairs will look like a damned fool screwed up the boat.

as long as you are starting over anyway, use straight gelcoat and either out ova over it or add an extra cost of gelcoat to be washed off.

here is a nice idiot proof set of directions:  

http://schrothfiberglass.com/GelcoatRepairPrimer.htm

 

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You don't use aerosil or Colloidal silica make gelcoat into a paste?

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I have learned the hard way, of doing gelcoat over epoxy, all of the amine blush must absolutely be removed for the gelcoat to harden.

 

warm water and plenty of soap is the best way to do this.

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