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Old 06-04-2009, 08:44 AM   #1
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Fiberglassing issue

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Howdy Guys,
I have been fiberglassing for well over a decade now and enjoy the process of manufacturing my jets. However, once in awhile I come across a problem that has my head scatching and perhaps someone can help out in solving this specific problem.

Once in awhile, I fiberglass foam core wings with the intention of using them to be the master plug to make moulds. I use general purpose polyester resin. Here's the problem, sometimes the resin simply will not cure hard is small areas. I clean off the offending area with thinners and re-apply new resin. More times than not, the new resin will take a day or two to cure, which really is aggravating when this is hlding up a project.

I have a good buddy who recently performed the same task on a pair of wings, he had tinted the resin as it's makes sanding easier to see the highs and lows. He contacted me and advise me of the same issue. We discussed it and thought perhaps it's the amount of tint that he used that casued the problem as he use way more tint than I had advise. He cleaned the surface properly, removing the offending tacky resin and reapplied resin. It took several days to cure, I think, but he eventally go the wings sanded.

On my wings, I used no tint at all. Also, the first layer of resin was used to adhere the fiberglass cloth, it being appled with a plastic card, squeeging the resin and reducing the amount of resin. The wing was totally sealed as a result. I then lightly sanded and cleaned the surface. On one wing, I applied the second coat using the squeege but on the second wing, I used a wide brush as it filled the exposed fiberglass weave better. I did this in the morning and during that evening I checked the wings expecting them to be fully harden and to do the other sides. Well, the wing that had the resin spread out with the plastic squeegee was hard as a rock, but the other wing had several areas along the wing root where the resin was still wet and sticky. I ceaned it and reapplied some resin with a bit more hardener. I then set up some hot lights over it and left it to harden. With the lights and additonal hardener, the resin should have kicked off rather quickly. The next morning I checked it and found that it was still a bit soft and a little tacky to the touch, but only in the exact area where it hadn't hardened it the first place, the excess around these areas was hard.

Has anyone had this happened and found a way to resolve the issue.

My resin is less than a year old in a 5 gallon drum ( I do a lot of fiberglassing ) as is the hardener.
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Old 06-04-2009, 09:33 AM   #2
Mike Emilio
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Re: Fiberglassing issue

Hi John, , , almost thought you fell off the planet. Haven't heard from you for a while.

I've experienced the same thing on occassion.

All the moulds I make are polyester as well. An epoxy mould would simply kill the pocket book $$. For bigger stuff I use a pressure pot feeding a Binks spray gun with catalyst injector, so I know I'm getting pretty good at the 2% MEKP ration, so I don't think the catalyst is the problem.
I also usually buy 5 gallon pails of resin from "Progress". One time I had a pail hang around for maybe 3 years before it was all gone, so I don't think it's aging resin either.
I've also thrown in up to 7% catalyst. It gets much hotter, but never had a curing problem, so I don't think it's a mix ratio.
When the resin was mixed, I'm sure you got the nice change in color from the pink to the darkish color, so the MEKP would have been relatively homogenous thru-out. Even if a tint was added, it would receive it's dose of MEKP in the mixing container.

When ever this happens to me, it's resin that was mixed manually by hand, and done in the basement workshop. But if it were done in the garage on a warm day, it never happens.

I made up a small batch to test. Part was applied in the basement, and part in the garage. The test in the basement was still rubbery after 1 day, but the same batch in the garage hardened up nicely.

Apart from the basement being naturally a bit cooler, I'm thinking it's got something more to do with the basement humidity, and possibly a varying thickness of resin.

A bare coat of the non-waxed laminating resin most often inherently will do this as well, and might take as much as 5 days to cure. On the last lay-up, I'll either throw in some styrene, or switch over to the waxed variety.

If your using a general purpose resin, the waxed variety? To set up a stronger humidity barrier, would adding a bit more styrene help?

Aside from everything above, , , your scenario seems isolated over the surface of the part. If it were epoxy, or making your own rubber tires, I'd jump on the band wagon of a sulfur contaminate.

As you know, Polyester is pretty stong stuff compared to epoxy. Was the brush used the same as you have used with no problems in your many years past? Is it possible that the mix reacted with the brush, and drew out some chemical contaminants from the brush bristles?

The only other thing I can think of would be the wet film thickness at the affected areas.

Last edited by Mike Emilio; 06-04-2009 at 10:44 AM.
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Old 06-04-2009, 11:44 AM   #3
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Re: Fiberglassing issue

Hi Mike,

Nope, still alive and kicking at least for now.

The polyester and the hardner are fairly new, have used about 1/2 of the 5 gallons so far and no problems.

Yep, I mix the resin really well and continue for a bit after the colour change as well. I can feel the cup getting warm so I know the resin is kicking off and the remaining resin hardens up in the cup rather quicly and dry So it's not that.

I do all my work in the basement all year round and eep the humidity fairly constant, at least I try. I also use the big contractor lamps to help speed up the curing.

This morning I did the ther sdes of the wings and pu he lamps on them, that was at about 10:00 a.m., it's now 12:30 P.M. and they are still tacky, but the resin in the cup is hard.

Hopefully by this evening, I'll get to wet sand these wings, if not, I'll wash them down, sand the surface and try using the moulding epoxy. I have the 20 minute working time stuff, so it just has to cure.

The main reason for using the glass cloth is so I can scribe the panel lines into the srface. Without the cloth, the resin simply cracks as the balsa isn't tough enough to hold the resin. Found this out when I tried scribing the panel lines in the vertial fin. But the vertical fin took te two cats of resin fine, now, I did hang the fin on a bungee cord suspened from the ceiling in the workshop, perhaps there was enough humidity difference or temperature difference that it made the difference. Hmmmm

Maybe, I'll give that a try as well.

Thanks for the info, I'll get ack to you on my results.
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