Gelcoat porosity

I mend the seam between the two sides of the hull and the stern/transom on new modern sailingboats. However, I´m having trouble with tiny needle sized airbubbles and a lot of it. I don´t know what I´m doing wrong.
The gelcoat is applied by brush, 2% catalyst, mix by stirring and let the gelcoat rest for app. 2-5 min to let air get to the surface. I put app. 5-6 layers on, hopefully in one day, but if not, I clean with acetone or sand before next layer depending on dust etc.
The working space is not secluded, since the boatbuilding company is quite small, so I have a dusty and temperature unstable working environment.
The surface the gelcoat is applied upon is newly sanded (280 grit) and cleaned with aceton.
Please, can someone give me some advice?

 

Resting won't let the air out, viscosity is too low, you have to vacuum then can apply directly. Multiple passes (takes longer) will work some of the bubbles out as well. Pre-cooling the gell coat will prolong gell time considerably, under 15 degrees C should remain liquid. Application by wooly roller may be faster, freeze the roller when not in use but not with food stuff to be able to use the same one over and over.

Devide gell coat in smaller quantities, cool in fridge overnight. Mix catalist, vacuum and keep in fridge untill required for application.

 

Air being trapped would be the result of a high viscosity, not low, and cooling the gel coat will raise the viscosity a great deal.

Make sure the gel coat you are using is designed to be brushed, I've never really had a problem brushing sprayable gel coat, but I have no idea of the brand or type you're using.

Try dropping the catalyst % to 1.5, or even 1.25, plus ask your gel coat supplier for another type of catalyst. Stir it well, but don’t mix air into it.

There could also be a problem with this batch of gel coat, sometimes it can be product related and there's no way to overcome it with technique or other adjustments.

 

Ok, thanks for your advice.
But, Fanie what do you mean by vacuum? And how? Also, considering temperature, we´re having a very cold winter here in nothern Europe, so some days the temperature have been down to 10 degreesC inside the yard and the gelcoat even lower. Therefor, instead, I´ve heated up the area and the gelcoat to about 15C, but I still get these pinholes.
Now I´m working on a yacht with dark blue gel. With white gel you can get away with some things, but on a dark blue, everything showes.
I´ve been experimenting with thin layers and thicker layers, no difference. Sanding between every layer, using a roller,, same result and it takes too long.
Could it be the effect of gas building up when it starts to cure? How can I prevent that?
I use the same type of gelcoat (Maxguards GN 10000H) and catalyst (Butanox) as the guys that makes the hull and deck from the molds and they apply the gel by brush also. They can´t help me by the way, I´ve already asked.

 

A friend of mine does castings, if he doesn't vacuum the batch after mixing there are quite a few bubbles. What we did was to make a vacuum trap, put the container with mix in it, close the lid and vacuum. The bubbles expand and raise to the surface a lot quicker. Vacuum is where you use a vacuum pump to suck the air out. Actually, it should not be a problem because when you apply the bubbles should come out by themselves now that I think about it.

The idea of working cool is just to prolong the mix before it kicks. Once it started gelling the work time is over.

Ondarvr is right, there may be a bunch of things that can cause the problem. You will have to play with different methods of application etc and see what cures the problem.

I used a roller when I apply and I just roll over the same area a few times. There were a few bubbles but not really bothersome. Keep in mind gell coat goes into a mould, if you want to paint an open surface you are supposed to use flocoat, a slightly different product that gives different results for the open application.

 

As I have it, the mould gets pollished, then waxed, allow wax to dry and wipe clean so the waxed surface is smooth and shiny. Then carefully apply red release to form the release between the wax and the gellcoat.

When the red release is dried it forms a thin rubbery layer, the first gellcoat is applied carefully and you pay attention to corners etc so no air gets trapped which will result in holes on the outside.

You are lucky, out weather here when hot gives you a few minutes to work in. Once the whole surface is covered you can apply heat in your case, it would be fantastic to be able to finish a whole area and the gellcoat has not kicked. The next coat should be applied before the first coat cured, see the user instructions.

I like and use Cray Valley's products.

 

Ah, thanks Fanie, for your time and advice.
I´ll take some pics to clearify how I go about mending the seam and, also, see if I can get the little buggers (pinholes) on a pic as well. I´ve redone the blue transom three times by now and soon I´ll have to put gelcoat on the whole transom if I don´t get it right.
cheers/Lena

 

Ok, I´ve attached two pics. The first one is supposed to show the needle sized pinholes/air bubbles. The second is of a not yet fixed seam on the transom of a 37ft boat. This is to give you an idea of what I´m doing and where on the boat.
I´ve read the Application Guide on CCP´s webpage (ccponline.co.uk), quite a substantial read, anyway, CCP have a troubleshooting guide with additional pics (thanks `tunnel´ for your tip) where I find pinholes rather than porosity to represent the cause to my problem.
I can´t figure out what I´m doing wrong. Everything I find on the internet mainly refers to spray gun application and tecniques, not brush.
Please, help me....
//Lena

 

Could there be water drops ?

 

Do you use recycled / cleaned brushes?

Which peroxide do you use? As you are in Sweden, you will probably be able to get hold of Andonox MEC peroxide from Syrgis (Norac Andos).

I suspect that you have moisture in your current peroxide. Can you do a test? Pour in a test tube a layer of 3 cm of peroxide, and 3 cm of styrene. Now with a latex gloved finger close the tube, and shake vigorously. If the mixture is hazy, there is definately water in your peroxide. If not, there COULD still be water in it.