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Working With Fiberglass

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hey. im helping a friend on a project, and we need to build some parts out of fiberglass. we thought about building a mold, but i know its very difficult to make a good mold, specially of a prototype.

so, my idea is to build the parts using plywood, and then, cover them with fiberglass and sand it to get a smooth surface.

can this be done?? is there a way to leave the outside of the fiberglass semi smooth in order to reduce the sanding?? and, will the fiberglass get stuck to the plywood properly, or will there be the risk of the fiberglass coming unglued and then i would end up having a bubble between the plywood and the glass??

i post this here because i think some people might have experience working with fiberglass. my only experience was when we covered a small hole on a boat, it has been holding great but stetically its not so good, as it was in a very inaccesible part.

thank you!

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Rhino......oh calling Rhino!


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Here he comes to save the day. Mighty Rhino's on his way...................................

So much for the theme song I plagiarized from a certain rodent.

Yes, you can make parts by using plywood, and encasing it in fiberglass. I'd suggest using Baltic Birch plywood, which you can get at the Home Depot. The plies are thinner and it doesn't have voids like other plywood, with the exception of marine grade. Marine grade is very expensive, and unnecessary since the fiberglass is the structural component. The plywood is only used to give the piece shape. Another significant benefit for using the Baltic Birch is that it is usually flat, and doesn't warp as much as other plywoods because of the thinner plies.

They have it in thicknesses from a quarter inch to half inch thick. Quarter inch should suffice for what it sounds like you want to do.

That's a general explanation. If you can tell me where that part will be installed and what it's being used for, I may be able to provide better, more thorough info.

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thank you Rhino!!

the idea is to build a modular hard top for a jeep.... me, and 2 friends studying and engineering on Industrial Design, have jeeps, and they want to build the hardtop since they only have soft tops... im a mechanical engineer but i am good with tools, while they havent built anything yet, so, they asked for my help.... so far i have designed most of the parts, thinking about fitting, protection from the environment, specially rain, and looks... im thinking about using 1/4" plywood for most of it, but some parts may require thicker wood, or 2 layers, to make it thicker...

the idea is to end up with something like the BRUTE conversion:


but, being able to remove the rear section, in order to be able to have the rear seat for the passengers. so basically, have a BRUTE cabin, but keep the jeeps rear section, so if we remove the rear module of the cabon, we still have a 5 passenger jeep... and, to close it, we will add a tonneau cover... so it would be something like this:


but, made out of fiberglass and with the looks of the Brute...

is it too ambicious?? we can have it made at a local fiberglass shop, but, it would be expensive, and, we wouldnt learn anything.... i believe in trying to do everything myself, sometimes i succed, sometimes i dont, but everytime, i learn something new...


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That's not a job I'd want to tackle as a one off piece, or even for two or three of them. The bugaboo is that you have to make it fit to the top of the windshield, and around the doors. To do that, you cannot simply mock up a cap then glass it. You have to take into account the thickness of the glass, so that it fits nicely around the windshield and doors.

Were I to attempt that, I'd use the foam that is used in fiberglass fabrication. Fiberglass resins will eat up styrofoam. Then you would meticulously shape the foam to fit to the doors and windshield. Then you would shape the countours you want onto the rest of the "cap". Once you get that done, you will use a product similar to bondo to fill the small pockets in the surface of the foam. Then you will need to prime and paint the "plug".

I used to know what type of paint is not attacked by fiberglass gel and resins. Once it is nicely painted you'll need to apply several (five or more) coats of mold release wax, so that your mold will release from the plug. You will then likely need to trim the edges of the mold to conform to the doors and windshield.

Then, the mold should be buffed and waxed (again, five or more coats of mold release wax) after it has cured for a week to ten days.

Finally, you are ready to produce the desired cap. Spray gel coat into the mold. Let the gel cure to the point where it feels tacky but does not stick to your finger. Depending on the thickness of the piece you may be able to laminate it in one shot. But, I'd suggest doing it in at least two steps. What can happen is that if the piece you are making is too thick, it will generate heat, and can distort (pull) the mold and you end up with a distorted piece. The same goes for making the plug and the mold. Do not. When I make a mold from a plug, I'll only laminate a couple of layers, then leave it overnight. The next day I can laminate four or five layers of material without a distortion problem.

As you can see, when you factor in the cost of materials, and your labor, economically it doesn't make sense. But, to do it as a challenging project, it can be done. If you or someone involved in the project is handy with tools, and has some handyman skills, it can be a fun project.

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i knew making the mold was difficult, but i didnt think it was THAT difficult... thats why i dont want to make the mold, i would rather use plywood and then glass it, leaving the plywood in there. i used to work at a shop where they did fiberglass parts for the Cessna aircrafts, and yes, it was a very laborious work to get it done right...

i havent checked the costs, but i think it would be cheaper than the $1750 they want for an original brute conversion...

on the design i have taken into account the fiberglass thickness, and i have discussed with my friends and they say it can be done, even though they have never attempted something this big, they have fiberglass experience on smaller proyects...

so, now im not sure if i want to attempt this... winter is coming, and having the jeep as a daily driver its not fun with temps in the 50s...

i already have the tarp setup shown in the second pic, but its not as wind or waterproof as i would like...

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