Gelcoat problem(picture attached)

I have come to this forum to see if anyone can give me advice to why my gelcoating is producing this effect.

I am gelcoating into a rtv rubber mould and i seem to be be doing everything right,the catalyst addition is ok and i am laminating as soon as the gel is tacky.

Any ideas what causes this....

 

Hmmm, difficult to say. Do you have a picture of the whole mould? Styrene (the stuff you smell) tends to float down. If it gets trapped in the mould, the gelcoat will not cure or not cure properly. That can be the cause for the problem.

You can either put the mould on its side, so the styrene can run out, or mildly ventilate. (put a fan in the neighbourhood).

Also try to go a little bit higher on catalyst, but do not exceed 3%. (or final cure can be jeopardised)

 

Thanks for the info.....the mould is an rtv rubber which produces a helmet sized item..i do catalise at around that percentage.

Also i do give it two coats of gel before i laminate and just to pick up on the point of laying the mould on its side..i still get this problem when the mould is layed in various positions while the gel is drying..

I'm mixing thoroughly and laminating as soon as the gel is tacky..

Many Thanks for any further info..

 

I saw the picture again, and it crossed my mind that it might also be pre-release due to shrinkage. How thick are the layers you put on? Try thinner layers.

 

I think i am putting on a layer approx 1.5- 2mm thick..what would be the correct thickness and its always puzzled me how would you measure it..thanks.

Oh and i am also brushing it on..

 

Holy Canneloni !!!

2 mm thickness? I guess we found the problem then.

At max the wet film thickness should be approx 0,7-0,8mm, preferably put down in 2 layers.

You can measure this with a wet film thickness gauge. Ask your supplier if they can send you one. If they refuse, get another supplier and I will send you a gauge for free.

Also you can guestimate the surface area, mix resin in an amount exactly what you need (a slight overhead for the brush) and then it is simple: That amount has to go in, and nothing less or more.

 

Many Thanks for the answers...i think i will reduce the thickness...to be honest i found that on one particular rtv mould(which is the one i was having problems with)the gelcoat sort of separates on the surface of the mould if you apply the gel at the correct thickness...have you any idea what might cause this..

Many Thanks

 

That type of defect is from under cured gel coat, I would go along with Herman's first post about styrene vapor collecting in that area and inhibiting the cure. Thick gel coat won't do that, thin or under cured will.

Wait a little longer before applying the glass, a fan or tilting it will help too.

 

Yes gel coat will separate because its silicone !! Happens always when you really dont want it to .
Why are you using a silicone mold?? is it the shape of the object that dictates of dont you want any joins to show ?
Definitely the gel-coat and the resin are shrinking-and pulling off the surface of the mold. Using Polyester resin by the looks of it !!
I have used silicone moulds for high re-leaf intricate patterns of wooden corner moldings when i worked in Television long time ago but used a mix of pigmented Casting resin and glass strands at low catalyst to keep the heat and shrinkage to a minimum ,worked perfectly .
Look at casting polyester resin to use with some chopped strand mat teased out and mixed with the resin and keep the catalyst ratio to a minimum !!
Another way to gel coat is to try using 50% resin and 50%gelcoat mixed and pour into the mold and roll round to coat and pour out the surplus . Gel coat with resin mixed gives a little less shrinkage than just gel-coat and to build a thickness you can do it twice and then glass the inside . Have to keep experimenting till you get what you are trying to do .

 

A 50/50 mix of resin and gel coat increases the shrink, plus it reduces the hide and UV resistance.

Normal shrink and pre-release don't look like that, there is far too much deformation, only soft gel coat can move that much and not crack.

For whatever reason that area is not cured well enough before resin and glass are applied.

 

Depends what country you live in as to what materials are available as to how they behave !! even just playing with different catalysts can help with shrinkage or lack of with gel-coats and with resins
50% CASTING RESIN AND 50% GEL COAT HAS FAR LESS SHRINK AGE !! Been there and thats what i used while i worked in New Zealand .
Not all countries have the same products and no two ever behave the same . As for the uv thing ! what do you want 100% product that is able to be painted or a gel coat finish with a reject rate or 90 % when it comes out of the mold !!. everything is a compromise ,even life !!
Shrinkage can be controlled to a degree as you work through the different types of resins and the range of catalysts combinations and temperatures for each and every one of the combinations .
There is no such thing as one polyester resin , theres a list as long as a roll of bog paper in any country you go to !!
Like saying you can have any color you like as long as its white !!
Problems are the same in any country Ive worked and the solutions to the problems are always the same but the products that are available are never the same to be able to solve those problems .

 

There is nothing in that pic that indicates a gel coat problem, or too much shrink. The issue is cure related due to technique or timing.

 

Thank you for all the info...i am really at a loss as to why this is happening..i do own other rtv moulds and i do not have this problem even though i am working through the stages just the same..

Admittedly i probably have been applying the 2 layers of gelcoat too thickly so i will reduce the thickness but i can remember purposely doing this one time and i got the same problem..

I am mixing the catalyst fully and i always wait till the gelcoat is tacky but does not mark the finger before putting on the next layer...

Many thanks for all the info but i am really at a loss..

 

Just to add some further info...the other moulds i have are quite open moulds sort of tray shape which i do not have this problem with..

The problem mould is helmet shaped and as you can imagine the neck opening is what i work through..i would imagine there is not much airflow inside the mould and going off some of the answers would it be just a case of using a desktop fan to blow into the mould as it is drying...


Many Thanks

 

The problem mould is helmet shaped and as you can imagine the neck opening is what i work through.

tell me about how you leave the mold when you have finished gel coating ?? It could be styrene fumes sitting inside the mold .:idea:
Tip the mold on its side so the styrene fumes can drain out or place the mold some where that get a movement of air continually over it to flush the fumes out from inside . Its some thing simple but can cause a lot of strife . The fumes retard the curing process and can even stop it from going hard like the rest of the artical .