How To Do Beautiful Enclosures With Custom Fiberglass

Feb 27, 2024

There are times when I feel the need to really make a mess. When I think of making messes with a degree of permanency, I think of fiberglass. I also really like the smell, reminds me of a simpler time in 8th grade shop class. But the whole process, including the mess, is worth it for the amazing shapes you can produce for speaker pods and custom enclosures.

Utilizing fiberglass for something like a custom speaker pod for a car is not difficult, but it does tend to be tedious when it comes to the finishing stages. If you have ever done bodywork on a car you know what kind of mess and effort I am talking about. In the video below, I make a simple speaker pod meant for mounting a speaker to the surface of something like a car door.

You can also use a combination of wood and fiberglass to make subwoofer cabinets that are molded to the area around them. You can even replace your entire door panel with a slick custom shaped one with built in speakers if you’re feeling adventuresome.

For a contoured pod I start with speaker rings cut out of Medium Density Fiberboard (MDF). These can be purchased or you can cut them out yourself. I typically buy some rings and then use them as templates for use with a flush cut router bit, or I go ahead and cut them out using a jig saw. MDF also makes a good backboard for the pod.

I could do a whole post on using a small table router to do things like recess the speaker or rounding off the speaker ring, however the same things can be accomplished using two different size rings or by hand sanding the ring to round it off. I do show my table router in the video but do not turn it on due to the noise and mess it would make in the middle of shooting a video.

Positioning the speaker rings to angle the speakers in the desired direction and height can be done any number of ways, I use little sticks cut from MDF to build supports for the rings using hot glue or epoxy to hold them in place. Hot glue? Yes, once the resin sets it will be the resin holding everything in place.

Next a fabric is stretched over the frame formed by the rings, my favorite being a polyester based fleece that can be purchased at craft/sewing stores. Being polyester, the fleece melds well with the resin and becomes fairly solid. In the old days we would use speaker grill cloth which was hard to get the resin to soak into or to stick to it.

The fabric is pulled down over the edges and stretched until no major wrinkles are present. A staple gun or even cyanoacrylate glue can be used to affix it, I use an air powered staple gun that puts down an impressive rate of fire. See the video for the trials and tribulations on pulling all of the wrinkles out and how to fasten it down.

It’s all about the polyester resin, which is used to impregnate a variety of materials including matting made of glass fibers and/or polyester fleece.

Unfortunately those little cans of resin you see at auto parts shops or even marine supply stores are not ideal as it may be old resin, resin of unknown quality, or even outright junk. The preferred way to buy resin is to find a source that talks about the freshness of their resin as resin has a shelf life of 3-4 months typically. I buy my resin from fiberglasssite.com and then write the date on the can. I buy a new gallon every summer, though I have hit a few years where I may not have gotten around to actually using it that year, don’t tell my wife.

As far as the type of resin I use, I usually use resins marked as layup or laminating resin, and almost always labeled as a “premium” resin (though I haven’t seen that there is an official classification). The implication is that a premium resin is from a production run and not the odds and ends of other production runs. I learned many years ago from watching a video by Robert Garza of Select Products that premium resins are red in color; green is an indication of a mixed batch of leftovers.

I have a big bottle of hardener lying around and then I also get little bottles with each order. The hardener is Methyl Ethyl Ketone Peroxide (MEKP) and is mixed in the ratios of 1-2% MEKP to resin, so for 400ml of resin I usually ad 1.5% or 6ml.

I buy my mixing cups by the box as well as my stir sticks. Somewhere around here is my favorite stirring screwdriver, but a throw away stir stick works also. Once the resin is properly mixed a change in color and opacity indicates when it’s getting ready to use.

I buy cheap paint brushes by the box, the foam kind also works. The idea is to soak the fleece with the resin until there are no light spots which indicate the presence of air bubbles. The resin can be stippled in using the brush or poured on directly and then spread with the brush making sure it soaks in thoroughly.

I use Rage Gold polyester body filler applied very liberally to the project. Then sand it down, add another coat, repeat. I use Roloc sanding discs in an air sander to remove the huge chunks of filler and then an electric orbital sander to smooth it out. Sanding sucks, wear your mask and eye protection and be prepared to get covered in a fine powder.

Depending on the finish desired you could use metal glaze and high build primer and repeat sanding until it’s smooth as glass, then paint with a high quality automotive paint. I went the other way in the video and sprayed it with a rocker panel textured spray product by SEM. The idea is that the texture helps hide some minor sins.

Custom fiberglass is also good for molding structures like sunken subwoofer panels and the like, where a very irregular surface needs to be turned into an airtight container for the sake of audio quality. One note is that a big subwoofer requires a lot of rigidity which might take an immense amount of fiberglass to achieve by itself, so it’s common to combine MDF and fiberglass (and acoustic dampening material) for the best effect

Commonly a glass strand mat is used for building up thicker structures. The mat comes in big pieces that are torn into smaller manageable piece and then impregnated with resin, building up the structure while eliminating trapped air bubbles. A paintbrush can be used to work the air out, but I usually use a roller made for the purpose with the downside that it’s one more thing that then needs to be cleaned. Production houses that do layup fiberglass professionally usually use a applicator that shreds glass fibers and mixes the resin all in one step while blowing it on to the mold.

A primary concern when getting resin anywhere near the interior of a car is protecting the car from all parts of the permanent mess of the resin. I tape down a drop cloth and then cut a hole and tape the edges. I then cover the entire exposed surface with the good green masking tape. I have seen some people coat the tape with mold release wax or spray Polyvinyl Alcohol (PVA) as a release agent. Instead of that, a trick I picked up from the Car/Audio magazines is to use spray adhesive and cover the tape with a layer of aluminum foil as it forms an excellent liquid-proof barrier and then separates easily when needed. Once removed the tape is peeled up leaving a clean original surface and a newly created fiberglass piece.

I do my own custom fiberglass because I enjoy it, (not the part with pinholes that mess with the finish) and if I do it right I have a custom look that could have otherwise cost some serious cash. It does take a little practice to get things right and getting two pieces to match can require some patience. But at the end of the day I usually get something similar to what I envisioned and that looks cool and unique.