Could someone please tell me about fiberglass gelcoat?


Master Member
A tutorial or a step by step process would be awesome. I can lay fiberglass matte just fine- but I want to be able to start with that nice, clean, smooth outer layer! Thanks in advance!

There are a world of possibilities with gelcoat. If you want to email me: I can help you on your way. I can get you books that put you through EVERY step of the way and I can help you with your questions as well. I work with it every day in mold processes and boat repair. There is even epoxy gelcoat but we won't go there. Anyway, let me know if I can help.

i remember a guy on the old board making a capn america shield with colored gelcoat?. i remember it was gelcoat.

man, it was SWEET. seriously a great skill to have in a hobby like this.

cept maybe the hobby of getting the occasional lotto ticket that wins. then that can fund this hobby

Brandon, I'm definitely going to drop you an email, but if you'd like to post some general info here as well I'm sure some other members would appreciate it!

Hi Douglas

The big thing with Gel coat is what ever you do dont apply it in thin coats. If you use too thin a layer the heat from the curing fibreglass will cause it to crack or craze. Also if the coating is too thin it will leave bare patches in the mold. You want a GOOD covering of gel coat & you wont have any problems.

Thanks Nathan,

I need to start at the beginning though, I'm afraid. Is gelcoat just a layer of liquid fiberglass resin brushed in without the matte, or is it a completely different product? If so, where do you buy it? I can lay down fiberglass matte well enough. The shape I am casting is fairly complex though (proton pack), I need the detail and smoothing help of a gelcoat layer...

Gel coat is a completely different product from Fibreglass resin.
For a start you can get it in different colours, ususally grey, white & clear can be obtained off the shelf from suppliers. Coloured ones can be made up by adding dyes to white gel coat. Your supplier should be able to sell you these along with the gel coat.
Gel coat is like a thick gloss paint & takes longer to cure than fibreglass resin, or at least the stuff i use does.
You want to really LOAD the brush up with gelcoat & use a 40/50mm brush. Use a cheap brush with white or clear bristles as if they come out they wont show up in the gel coat. Once your done just bin the brush.
Work the gelcoat well into the corners as air bubbles can get trapped there.
Gel coat is that thick it will even stick to vertical/inverted surfaces but it sometimes starts to come awat from sharp edges. Dont worry if that happens the fibreglass resin will seal it.

Once the Gel coat has hardened give it half a day to fully cure as the fibreglass resin will start to break it down if you apply it too soon.

Hope this helps
I know that some people on the board mix body filler and resin together to make a cheap version of gelcoat. I have done this, and it worked very nice...


First of all I don't want this taken the wrong way. We have had far too many wars here lately and I don't want you to be offended by my corrections to some of what you have written. We all want the best from our work so I hope you understand.

First of all, you are correct about not wanting gelcoat to be too thin, but some of the reasons listed are incorrect. Gelcoat is a resin, only it is higher grade, has longer elongation, has pigments and resists UV rays much better. Both resin and gelcoat are made the same way, polyesters and styrene monomer. Both use the same catalyst with the same percentage to cure it and both, when done correctly, will take the same amount of time to gel and cure if their gel times are compatible. Even with varying gel times they will cure in about 2 hours at 77 degrees F.

Cracking of gelcoat comes when it is applied too thick, not too thin, in excess of 25 mils. Optimum thickness is 18(+/-2) mils. Below 12 mils and it will under cure and will have fiber print through. Resin will not break down gelcoat when it is cured, thick or thin. Paint on a car is 4 mils thick for reference.

Crazing is something that happens in resin and not gelcoat. Cracking or checking (mud cracking) occurs in gelcoat and is caused by gelcoat being too thick, blistering, evaporated gasses behind the gelcoat that expand and blow out the surface, and stress. If gelcoat is too thin, it will flex with the fiberglass easier. But thin is bad right? If thicknesses do not exceed 22 mils it will still flex with the glass well enough and not crack, otherwise the stress of the flex will crack the gelcoat. Fiberglass must flex, otherwise a boat would shatter the first time you take it out.

I recommend spraying gelcoat onto a mold. Brushing can be done but the results will not be mirror of the mold surface. And if you want an even coverage then spraying is the way to go.

After I spray gelcoat onto a mold I start my skin coat of fiberglass about 45 minutes to an hour after spraying. You can start in as little as 35 minutes after applying gelcoat if you are in a well controlled 75 degree environment. The norm is when the gelcoat cools back down to room temperature. Waiting too long and the skin coat will not adhere well enough and delamination will occur down the road. I always follow that rule. If I wait for a long time between coats of fiberglass I can always sand it to rough it up. The gelcoat should be tacky when you start laying the skin coat.

A supplier can provide any color of gelcoat needed. For exact matches, like boat matches, I use a place in Washington that supplies factory colors.

I think some of the reasons your gelcoat is taking too long is either the amount of catalyst you use (should be 1.8% at 77 degrees), room temperature at application, applied too thin, not mixed well enough before mixing catalyst or old gelcoat. You might also have incompatable catalyst.

Hope this helps. Happy prop making!

Hi Brett

Dont worry i didnt take it the wrong way.

Im no expert with Gel coat, i was just trying to help out by pointing out things that i have found while using Gel coat.

What i meant about the longer drying time is that Fibregleass will start to go off very quickly without much warning where as gelcoat has a longer usable time and gives you more of a warning when it starts to solidify. Living here in sunny Scotland things take a little lnger to cure.

Thanks for the advice though its helped me understand gel coat a bit more.

Yeah, living in an environment where there is higher humidity will definately effect applications with plastics. I live in the second dryest (is that spelled right) state in the US, so I don't have that problem, fortunately.

But what sucks about living here right now is 2 days ago it was 79 degrees F. Now it's 36 degrees and snowing! Just north of me, about 35 miles, there is 9 inches of snow on the ground. Hey, we could always have the winter olympics again! I'm sure all our venues are stacked up with the white stuff. <g>

hey Westies-er Douglas

Why are you gel coating your proton pack ? I think the effect would look great BUT unless you want that darth vader look to your pack, it would look waaaay to glossy for a proton pack. I am from the school that the packs were low sheen at most, at least from my observations of seeing the real thing in D.C..

For my Fiberglass packs I simply brushed on 1638 (or master cast) resin first, mudded it (fiberglass resin and cabosil) and fiberglassed it. The end result was beautiful as the 1638 really picked up on just about EVERY surface detail of the silicon mold (even light dust !). Best part was, glassing the pack only took 3 hours and about 2 hours to dry if on a sunny day !
Nathan, Brett:

Thanks so much for your input. What kind of equipment do you use to spray the gelcoat into a mold? What is the cost of the gelcoat supplies, on average? This is a great quick education for me, I really appreciate it!

Actually, whatever your surface on the mold is, that is what you get on the part. So, if you have a satin or dull finish on your mold, the part will be dull. You can spray PVA on a shiny mold, though, and it will pull dull (those two words look funny together like that).

Douglas, I use a pressure pot, gravity feed, siphon feed, or my gelcoater (It's a big piece of equipment that takes the gelcoat directly from the drum and mixes the catalyst in at the gun tip-very expensive).

I recommend a pressure pot or siphon fed gun. You can get really cheap ones from Harbor Freight. Make sure your pressure is at 35-45 psi at the gun tip. For the pressure pot, no more that 10-15 pot pressure is needed. You need good atomization or you can end up with porosity. The tip should be no less than size 18 on the siphon gun. You'll need lots of fluid coming out in order to get about 5-6 mils in each pass.

Gelcoats usually run about $50-$100 a gallon. If you are going black, it will be closer to $100. One thing you need to know is black must be catalyzed at a higher percentage than light colors. Catalyze at 3-4% by volume. Catalyst comes in 1 gallon containers and usually runs about $25 a gallon. Clean up immediately with acetone. If you need to thin the gelcoat, reduce with about 5% styrene monomer.

Hi GotMaul,

The gelcoat was not intended to be the final exterior of the pack. The surface can only pull as smooth as the master, and the shell is to be painted when it is pulled from the mold. A brush on resin would be just as good, better even if it is an easier application and just as durable... I am "exploring my options!" Please send me some more info on the products that you used if you get a chance! Thanks Jose,

hey douglas, okay, I got your idea !

If you are going to be painting over the gel coat, wouldn't you be defeating one of the main purposes of gel coating.

As far as supplies go, the type of brushup resin I use is the 1638 or 1630 or Master Cast- all are the same thing, all can be had from any place that supplies this stuff. You might want to look up an FX place like F/X solutions (down here in CAli). The mudding process is a combination of Cabosil (dried silica) and Fiberglass Resin (which can be found in Home Depot or something place like it). The fiberglass can also be found there too.

After that, there's not much to it.


I'm not sure I understand what you mean? I want to use the gelcoat to have a hard and perfect surface layer on the casting... Am I missing something about the fundamental nature of gelcoats? From what I understand, the process you're describing would achieve the same surface. Would it be as hard and durable? Are you saying that this process is much less expensive than gelcoating, and gelcoat's finish is something that I should pay for if I'm painting it- that the rest of the properties will be the same fundamentally?

Agreed, this is a great and educational thread. Thank you to everyone who's posted so far, and to those of you who have PMed with additional information!

jose's method looks and works great! I have seen the finished pack- and it has a nice smooth gray resin outer layer and then a nice hard backing of fiberglass for strength!

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