Hey guys, this is my first post. I have been making a set of moulds to make a carbon fibre wing skin. I started with cutting my pattern out of high density blue foam. I then coated it with a couple layers of coating epoxy with a few scoops of microbeads thrown in to make it thicker and sandable. After sanding it down to a nice smooth finish, I cleaned the surface with the Easy-Lease mold cleaner. I then applying several coats of Easy-Lease release agent (letting it dry for about 15 mins between coats). I then went ahead and applied the gelcoat for the uni-mould process. I put a second coat on after letting the first dry for a few hours. I then let it cure over night. The next morning these wrinkles appeared in the gelcoat right in the middle of the surface. I figured they were only on the exterior. I then proceeded with the coupling coat followed by the reinforcement. The next day when I pried the pattern out there were these worm holes where the wrinkles had been in the gelcoat. They appear to be air pockets but when I put the gelcoat down (both times) the was a generous even coat. I've been following the recomended ratios for the catalyst so I'm right stumped to figure out what I did wrong. I tried a second go using the same process but with waitng a little longer between the steps but I got the same wrinkles in the gelcoat again. Can you help me?

Hi Caden,

Thanks for your post and an interesting problem to 'debug'. Many people who use polyester gelcoats (which the unimould tooling gelcoat is similar to will testify to the frustrating problem that sometimes occurs where if the gelcoat has not cured far enough when you either re-gel or put on your baking resin you can get a reaction where the new resin (or second coat of gel) attacks the first coat causing it to blister or 'orange peel'. I've never had any such problem with the uni-mould system myself but certainly in the uni-mould system my experience is that the gel cures slower than you would expect and the resin cures faster than you would expect. If you waited a 'few hours' before you put the second coat on then my guess is that this delay was not long enough and that the second coat of gelcoat has attacked the first uncured layer causing the blisters.

You don't say how many hours a 'few' hours is but that first application of gel could take 8hrs easily (depending on your catalyst ratio, the ambient temperature and the amount of ventilation you have in the room). If you re-gelled after only 3 or 4 hrs in cool-ish conditions or with little ventilation then I could well imagine this happening.

I guess that you couldn't see any wrinkles after the first application of gel but you could after the second?

Even without hearing any more info I'm 99% sure this is the explaination. The solution is to ensure that any polyester or vinylester gelcoat is cured well into its safe zone (I just made 'safe zone' up but I want to differential between a B-stage that you would take an epoxy too which is quite different because you would never get this type of reaction if you re-gelled epoxy too soon). This 'safe zone' means that it's well cured and feels hard but is nice and green (i.e. fresh with a surface that will still be responsive when the next coat is applied. In the case of polyesters and vinylesters this is around 8-48hrs.

Matt Statham
Easy Composites / Carbon Mods - Technical Sales

Matt,

Thanks for the help. It's funny 'cause I was going to try moving quicker for my next one and apply the coupling coat after 2-3 hours. You had it right with the conditions I had it curing in. It was around four hours between coats of gelcoat. For my third attempt I am going to try just one coat of gelcoat and once again leave it overnight. My second part came out this morning with more holes than the first one so I'm working now to get them filled and sanded down. A little elbow grease and hopefully it will look okay. I'll let you know how the next one turns out...

Caden

Attempt number 3 was much better with no wrinkles. I went for just applying one coat of gelcoat however when I got it out and polished up, you can clearly see several places where the brush strokes were too thin. I hope this won't be a problem for using the mould with my prepreg. For my fourth attempt I went back to doing two coats of gelcoat although this time I let it cure overnight after each application. It came out with only two very small wrinkles this time so I think I've got it worked out. I still wonder what would happen if I had moved faster with the two coats, giving only a couple hours between each, then going straight to the coupling coat on the same day. I read a post from a guy that did the whole thing in one day so I guess it would be worth a try. Maybe on a smaller part.

I think I am starting to notice, if your clay you use for making the mold has been used a few times before it seems to be more susceptible to worming or alligatoring. Must have something to do with the clay absorbing something.

 



If Brute Force Isn''''t working your not using enough...

I think I am starting to notice, if your clay you use for making the mold has been used a few times before it seems to be more susceptible to worming or alligatoring. Must have something to do with the clay absorbing something.

Hey Don,

Where are you thinking clay is involved in this (do you mean the mould making putty? - if so, how can you use that a few times??)

I still wonder what would happen if I had moved faster with the two coats, giving only a couple hours between each, then going straight to the coupling coat on the same day. I read a post from a guy that did the whole thing in one day so I guess it would be worth a try. Maybe on a smaller part.                                                    

Hi Caden, Im glad the new attempt went much better. RE the above, I'm convinced it will go badly but give it a go for your own peace-of-mind and just to prove that undercuring is where the problem lies. A double application is favourable over a single so it's worth figuring it out. We get lots of feedback about how slow the Uni-Mould gelcoat is, especially at lower working tempertaures, so use the higher catalyst level and ensure plenty of cure time between coats to avoid the reaction.

--Matt

Matt Statham
Easy Composites / Carbon Mods - Technical Sales

i think what he means is modelling clay or wax used on the plug.  ie if hes reused the clay off an old plug, it may well pick up contaminates.

The other thing I would suggest is are you sure you mixed thoroughly. Mix well in one cup and then transfer to another cup and mix again with a new stick. I used to get all kinds of problems like you describe until I made this my practice.
Hope this helps.
Fred

i am unsure if this is related but am tossing it out on the table so to speak. i had some polyurethane paint
for the engine bay of our F16 jets, and it was sprayed on cold sheetmetal and wrinkled immidiately. it did cure
and i wet sanded it smooth and fixed it. paint and epoxy obviously are not the same, but when things are made for
a certain temperature range and you get on that borderline--weird things happen,
 my thought is , perhaps cool work conditions and cold materials could be part of the problem?

CarbonFiberCreations

So you can apply a second coat of styrene based gelcoat even though its pretty much cured? I thought it had to be just tack free so it will bite.