Fibreglass : Mold and Enclosure Making : How To and How Not To

So if you are thinking of making your own fibreglass enclosure, great! It’s heaps of fun. But it is a MASSIVE amount of work. I totally underestimated just how much work it would be, but the process was satisfying and I learned a lot.

There are a few different ways you can create a fibreglass enclosure, but I am going to focus on the way that I did it here, and hope that others can share their experiences as well.

PART 1 - Making the Mold

I decided that I wanted to make a negative mold for my enclosure because I felt it was the best way to achieve the narrow gaps between the segments and also a good finish on the outside.

To make the negative mold, I need a positive buck! I made mine from chipboard and ply. First I traced the board onto some paper, and then designed the segments to be large enough to hold the required components.

Once I had my segments, I decided that I wanted the gaps to allow wires to get between segments easily, so I 3d printed a small spacer to put between the segments. Everything fit so I shaped the segments and painted them black for a rough idea of how they would look and to more easily see any spots that needed sanding… The more sanding you do at this stage, the less you should have to do further down the line.

I should have just gone straight to the priming stage, the black paint was unnecessary. I used a primer filler as it supposedly fills small imperfections.

I also wanted to add a little flair to the enclosure so I 3d printed a couple of emblems. These were problematic and made the whole process a little more laborious but I think they look good. I used spray adhesive to hold them on the bucks and primed over them.

My suggestion would be to make sure that these are VERY well stuck down before you prime. The primer will try and lift these off the buck as the paint dries.

I stuck the bucks and spacers down to the deck with hot glue.

Hot Tip!!! Before you stick anything to the deck, first carefully mask the OPPOSITE side of the deck. Then you want to mask the sides, with your tape wrapping over the masking you just did. Finally you want to mask the side that the enclosure will go on, making sure that it wraps over the sides. Also important to note that the finish of this masking will end up on your negative mold so make sure it is NEAT.

You might be able to see little ripples in the tape on mine, I needed to sand these out at the end.

Finally, I needed a way to form neat corners from the deck to the buck. For this I used plasticine clay.


The plasticine was oil based so in order to smooth it out nicely, it is beneficial to use oil. I read about people using mineral oil, but i didnt have that so i used peanut oil :rofl: it worked fine. The heat gun was also super handy to warm up to plasticine and make it more pliable. I also used the paddle pop sticks to help with this.

Once I was happy with my seam, I cleaned up as much of the oil as I could and applied the release agent. This allows the finished mold to easily release from the buck. I used PVA release agent, but there are other methods like wax, aerosol release agents and probably more. I brushed mine on, but I think it would be better to get a sprayer that does a real fine mist. You need several coats of this release agent and you need to let it dry between coats.

Finally I was ready to start on the actual mold! I decided to use a gelcoat as my first coat, to make finishing the mold a little easier. To the best of my knowledge, all gelcoats are polyester based. I read in a few places that you can’t successfully put polyester gelcoat over epoxy resin. But I couldn’t find any information about using polyester gelcoat in an epoxy resin layup. So I used it and it worked fine.

Edit: I have had further advice that it is not a good practice as there is virtually no chemical bond between the two elements.

Once the gelcoat is applied, it needs a couple of hours to tack off before any glass or resin can be applied. You want the gelcoat to be hardened but still tacky when you start applying glass.

I filled up the segment gaps with fibreglass fluff and resin. You can buy this, but I didn’t have any so I finely chopped a bunch of scraps. I used a paddle pop stick to jam it in there and push the resin in too.

I was using quite heavy weight chop strand matting for my enclosure and it was difficult to get it to conform to the corners, so I cut it into sections to make life easier.

The laying of the glass and resin was definitely the hardest part of this whole process. You need to make sure that all of the glass is thoroughly wetted with resin, and also that there are no air bubbles. I ended up with a couple of bubbles in mine because I was in a rush and it was super painful to fix later. You can see in the photos above that some of the glass strands are still white and not wet, this happened because the resin I was using had started to catalyze and went off before I could thoroughly wet the glass.

My advice is to make your resin in small batches, to avoid losing large amounts of resin because you took too long! I have been told that if the resin is in a dense environment like a cup, it will catalyze faster than if it is spread out like on a plate.

Edit: Apparently the reason is on a molecular level, molecules are further apart on a plate or large flat surface. Worked well for me.

At this stage, i thought I had completely fucked my mold and I needed to go out, so I didn’t go any further. But for you guys, the advice would be to let that layer tack off a little and then apply another layer of resin and glass.

Once cured, you need to use a plastic or timber wedge to try and separate the mold from the buck. I couldn’t get a wedge in anywhere so I flexed the deck. This did a great job of separating the deck from everything, but the bucks were now stuck in the mold.

I used little wedges and I put a screw into the bucks so I had something to grab and pull on. It worked very well.

My mold turned out great! Apart from it being a little flexible cos it was only one layer of fibre :rofl: and the air bubbles definitely caused me some grief, but nothing that couldn’t be fixed. My mold did not look this clean when it was separated, it was covered in the PVA release, which just cleans up with water. It also had a lot of the plasticine stuck to it.

I cut the excess from the mold with a multi tool with the grout remover attachment. This was slow compared to using an angle grinder, but the dust was controllable and its pretty hard to make a big mistake with the multi tool. While I was sanding down the edges, i discovered a soft spot from one of the air bubbles.

I chopped out as much of the loose gelcoat as I could with a knife, and then filled the hole with resin.

I have sanded the crap out of it and discovered another little air bubble/hole so that is currently filled with resin and drying.

I sanded everything down to 240 grit dry, and then wet sanded at 400.

PART 2 - Making the Enclosure

After a lot more filling and sanding cos I kept finding air bubbles I was ready to make the enclosure.

Using this aerosol spray release which may not be the best idea because I couldnt find anything written about it anywhere. After it dried, it left a powdery, gritty residue which I wiped off with a dry rag.

Using a polyester gelcoat again, but this time it’s a neutral color and I used black pigment. I wanted brush coat but they sent me spray coat so I dug this shitty old spray gun out of the shed and cleaned it up.

It was a bit of a mission cos the gelcoat was so thick and hard to spray but I got there in the end and it is a much better and more even coat than the brush coat.

I also pigmented my epoxy to try and mimic something hopefully close to an @eBoosted enclosure. I started the layup with 200gsm cloth, starting in the bottom of the middle segment and working my way out.

After the cloth I used 450gsm chop strand, cut into the shapes of the segments but a little larger to wrap up the sides. You can see where I cut the corners, this makes it a lot easier to form into the mold and also doubles up material in the corners :+1:

Added a final layer of the cloth and smoothed everything out with the special little rollers. It’s also important to use these rollers on every layer to eliminate air bubbles!

Now I don’t have any peel ply. And I am really regretting that. Peel ply is used as the final layer and rolled into the previous layers. However it is impervious to the resin and can be peeled off once the layup is dried, leaving a nice flat surface that requires minimal finishing.

Rather than peel ply, I used black builders plastic and pushed this by hand into all the nooks and crannies as best I could. I then filled up the segments with rags and wrapped the whole thing in a crappy kids sleeping bag before it went into the vac bag…




Great work :slight_smile:


Great Stuff ! Can’t wait for the finished product! :smiley:


Since I just finished a fiber glass enclosure I know the amount of work. Well done. It will look so good at the end adn you can make more enclosure out of the mould.


So much work! Haha. It’s for an Evolve GTX, so hopefully I can make a few more and get some dollars back :+1:


I hope so…this process is endless…the more you sand the better it gets but takes forever :rofl:


That’s why I wanna do all the sanding in the mold now so there’s less to do on the finished enclosure(s)!


Since in my case I wanted to do one enclosure only I thought to go for a positive mould. How much sanding would it need? Well 3 days later I understood thst wasn’t a great idea. Took forever to make it smooth :joy:


I am having flashbacks…


I was just reading your thread a couple of days ago, I can see how much effort went into it!

I like the idea of being able to reproduce it reasonably easily, plus I really wanted to learn how to do it!


Have you made something similar? Got a thread on it?


Simpler mold. I didn’t make a post dedicated to it but had some posts about it somewhere on builders


Had a quick look but couldn’t find it… did find this though…

@BigBen do you use gelcoat on all of your enclosures? I have been told that it doesn’t flex well and is prone to crack, have you encountered anything like that?

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I remember that.

Found yours

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I think I have some stuff embedded in the Tico build thread too. But I didn’t do a write up really.

Yes gelcoat, like a boat to so many other production fibreglass parts.


Update. Its been 5 days. What the hell have you been doing with your time?? :stuck_out_tongue_winking_eye:


So much filler :man_facepalming::man_facepalming::man_facepalming: ended up with a few more air bubbles than I initially thought, filling and sanding all week.

and the fibreglass mob sent me spray grade gel coat rather than brush grade so I’ve had to gather stuff for the spray gun. Should be good for layup tomorrow!

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I’ll be here tapping my foot. Looks good so far. Never worry about filler. Our resident expert Alan uses plenty of bondo in his creations.

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