Gelcoat pearls when i try to brush it.

Hi, i'm trying to use a gelcoat made from pu resin mixed with aerosol as a thickener. My mold is waxed with hard wax that is carnauba based. My problem is that when I try to brush it on, shortly after lines appear and the surface of the mold can be seen, in other words it sort of pearls. Can anyone tell me what i'm doing wrong?
Also does anyone know what is used to colour pu resin?

Thanks.

 

Man, I've never been able to brush gelcoat without seeing streaks of the mold peeking through. But I've usually brushed parts where the finish wasn't important. On the bright side, when I added a bit of gelcoat to tint the resin for the first layer of glass, the streaks were barely noticeable.

When I've brushed a part that DID need a good finish, I let the gelcoat cure completely, then just before the first layer of glass was applied, I brushed a second layer of gelcoat mixed hot. The resin for the skin coat should also be mixed hot, and applied while the second layer of gelcoat is still wet. Y'see, if that second layer of gelcoat lays uncured against the first layer for very long, that first layer is certain to alligator.

 

Alligatoring means you have undercure in your first layer.

check:
-catalyst level
-cobalt level (if the gelcoat comes not accelerated)
-temperature
-thickness of layer

When gelcoat pearls off, it can be 2 problems:

-gelcoat too thin (not likely)
-wax not polished good enough

Also the gelcoat might be inferior in quality, there are additives that help prevent pearling.
Applying a layer as nice and smoothly as possible (roll application) also helps. Apply the layer thick enough. 0,4mm is the bare minimum, check this. Also for boat, you must apply a second layer, for a film thickness of 0,7mm cured.

 

Thanks for the responses,

I should have added in my first post that im new to GRP and have never done anything with gelcoat, even though I have lived and sailed on grp boats for a large part of my life

I polish the mold using a standard hand held car polishing machine and then go over it again with a soft cloth, im not sure if this is the best way to polish wax, but it gets so smooth its hard tell if there is still wax on the mold. I use the catalyst as instructed, 2 parts to every 100. I have also tried mixing different levels of viscosity ranging from very thin to a bit thicker then golden syrup, and brushing on different thicknesses of coat, but even if I apply more it still pearls soon after brushing, building lines that are approximately 0.6 to 1mm thick, when right next to them there is literally nothing, this usually happens in the direction of the brush strokes. I have tried spraying with an electric paint gun but it would still pearl into blobs. I have also tried using aluminum or iron powder to thicken the resin, but got similar results. All laminating was done with a room temperature between 18 and 24 Celsius.


Thanks for the advice about alligatoring, I was also having some trouble with that. I'm not really sure what is meant by mixing the gelcoat hot? more catalyst or to heat up the resin? Also those additives sound promising, where can I find out more about them?

I am very grateful for all responses.
Jan paul

 

By "hot" I meant using the maximum amount of catalyst for the temperature you're working in. You don't want to over-catalyze, but if I'm applying a second layer of gelcoat, I use about 1/2% more than a reasonable person would. Of course, if it all goes to hell you may have to start all over.

You can also use the poor man's autoclave. That is, roll the mold into the sunlight as soon as it's gelcoated, but not if it has started to kick (or gel).

Real men brush their gelcoat. Any sissy can spray it, but if you ever decide to get soft, find an automotive primer gun (bigger hole), a pressure pot, and a compressor that will keep up with the gun. (You don't want to run out of air in the middle of a project.) I'd go through about a gallon of gelcoat in one operation, so I divided that gallon into 4 cut-off plastic milk jugs with the handles still attached. I'd line 'em up, catalyze one hot, put it in the pressure pot and spray it, then catalyze the next one in line...you get the picture.

If you spray, you need to be careful about thin spots next to thick spots. Thin spots may alligator anywhere on the mold, but if you have a thick blob of gelcoat adjacent to a thin area, the difference in their curing rates MIGHT cause pre-release from the mold. You want 6 mils, then stop spraying no matter HOW much extra gelcoat you mixed. (You should be able to get a mil gauge from your gelcoat supplier.)

 

From what you said it sounds like you're mixing your own gel coat out of resin.

Some people do this (or used to do this), but here are many other ingredients that are used in gel coat to make it spray (or brush), level and flow correctly, you won't be able to get these items.

Buy some real gel coat formulated to be applied with a brush and your problems should go away. There is also the possibility that the wax you’re using has some ingredients that aren’t compatible with the resin/gel coat mix you’ve created, gel coats don’t like silicone.

 

Hand brushing is a delicate art

You need to use plenty of gel and long strokes and the last stroke brush back into the coat .
Remember unlike paint it thickness you are needing .
Also you need a long soft haired brush and hold it almost with two fingers and trail it so you leave a smooth coatbehind the brush .
Those marks you are getting is because theres not enough Gell coat on the surface and the wax is really doing it job .
Another thing you can try is using pva release agent over the waxed surface before you start gel coating , Make sure the pva is 110% dried before you start brushing .
Always brush in one dirrection where ever possible and last stroke brush back into and lift off gently .
Did you buy brushing gel or spraying gel ?? The brushing a little thicker than spray and has a little more and better fixotrophic .
If you are trying to make you own DONT !! resin and pigment along with aerosil is not good , one the areosill dosent mix properly , and you could and probably will get pigment separation when you brush it .
Lastly there are no UV inhibitors in you mix and it will breakdown very quickly if exspossed to the sun light after a short period of time .
There are no short cuts when you are using gel coats and resins .

 

Aaggh... sorry, should have made it clear that I'm mixing my own galecoat. Somewhere on the Internet I found a page of someone who apparently is an old fox in the kayak building industry, he wrote that in his days galecoat was a bad word and he always mixes his own from the same resin he uses for the kayaks. So I thought I'd save an euro or two and do the same, sadly he did not give any instructions as how its done.

I have tried using pva but got the same streaks as with wax plus the imperfections on the surface cosed by the pva.

I guess I really should go buy some brushing gelcoat to see how it behaves. But I would still like to know how I can mix my own, even if its not going to be perfect it would still come in handy for parts that don't have to be 1A.

@Tunnels. I thought that pu resin was uv stable?


I really appreciate the responses, and am learning a lot. Thank you.
Jan Paul

 

The formula for a gel coat would change with each resin you planned to start with because it would depend on what type of resin it is, how it was formulated and what thixitrope they used. You may be able to come up with something that works in one resin and not the next.

I would say that your current mix is probably low in viscosity.


Most Gel coats rely almost 100% on the pigments for UV protection, base resins are also chosen for good UV resistance if that's what's desired for the application.

Polyester resin is more resistant to UV than epoxy, but it still needs all the help it can get to prevent degradation.

 

The point you bought up about PVA leaving surface marking can be illimintaed by spraying the pva rather than wipeing or brushing it on . Also thinning it a little with water and spraying 2 or 3 very thin coats allowing it to dry completely between each coat . Point to remember is if you can see it on the surface you have to much on , A Little is better than a lot in this case !!