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Discussion Starter #1 (Edited)

After posting pictures of my first fiberglass subwoofer enclosure, I received a lot of questions and requests for a detailed how-to on the process. It seems that there are plenty of people who - like me - want to try their hand at fiberglass, but don't really know where to start. Well, neither did I!

Hopefully, I will be able to include many of the little details that the pros frequently omit, but newbies really need to be successful.


My car is a 2002 Mazda Protege 5. Some of the procedures will be specific to this model of car, but the vast majority of the information here will work for any vehicle. Specifically, I built this enclosure to fit the passenger side of the hatchback trunk, to fit two 10" subs. The passenger side of the hatch is a bit larger than the driver's side, and also has the unique feature of a removeable panel behind which the jack is stored. By removing this panel, I gained slightly more airspace. This also allowed me to use the jack mounting bolt to secure the final enclosure to the car. I can suggest that in whatever location you choose to fit an enclosure in your car, you take the time to plan out how it will be fastened to the vehicle, as well as what you'd like the final product to look like. Take measurements, make sketches - whatever it takes to have a decent plan in place. And then know that parts of that plan will change as you go along, but that's okay!

Be aware that this is MESSY stuff. Protect yourself by wearing long sleeves and gloves. Getting resin stuck in your arm hair is NOT fun. Protect your vehicle with old sheets or other drop-cloths. And when working out of the car, protect the work area. I'm already hoping I didn't burn my security deposit on my apartment by dripping fiberglass resin all over my concrete terrace. Keep it clean!

Free distribution of this write-up is encouraged. Any commercial use is strictly prohibited! Either give this information for free, or don't distribute it at all. Selling of this document, or any other for-profit use, in part or in whole, without the express written consent of the author is completely prohibited.



Nearly everything I used for this project was purchased at Home Depot. Here is a basic list of the materials used and approximate prices:

  • Fiberglass Mat & Cloth: under $30 9 sq/ft (1 yard) packages. I used approximately 6 packages, at under $5 each.
  • Fiberglass Resin: approx $50 Buy it by the gallon (I didn't). I also wasted a lot of resin during various stages of this project. Buying by the quart costs a lot more. Each gallon container is about $25, and it's better to have too much than too little.
  • Brushes: $5 Get the cheapest kind you can, as they will probably not be reused. I used the foam brushes, but I would recommend using regular bristle brushes. Get some of each if you want to experiment. Let's say, 10 brushes @ 50 cents each. If you choose to try and save your brushes, put them in acetone immediately after use.
  • Masking Tape: under $10 A couple of rolls of wide tape should do the trick. I would suggest using mostly the blue painter's tape (again, I did not), but a roll of the stickier stuff could come in handy also.
  • Mixing Buckets At 75 cents each for the plastic kind with volume marks on the side, buy 5 or more.
  • Mixing Sticks Just like the paint jobs on Monster Garage, these are freebies. Get a stack at the paint counter.
  • MDF: about $10 Medium Density Fiberboard
    I used 1/2" and 3/4" thick MDF for this project.
    You don't necessarily have to buy big 4' x 8' sheets. My local Home Depot has 2' x 4' sheets as well, and these are cheaper and easier to transport. One of each thickness oughtta be enough.
  • Wooden Dowels: under $5 Scraps of MDF or other wood can be substituted for the dowels.
    I bought a couple of 6' lengths of 3/8' diameter dowel. You may use more, less, or other sizes for your project.
  • Wood Glue: couple bucks. Yellow carpenter's glue is great
  • Fleece or other fabric: about $15 I purchased a polyester/cotton blend at a local fabric store. I got 3 1/2 yards, which is a lot - but I also used it as a final covering for the enclosure when it was completed.
    Make sure your material is stretchy. I have also read about the use of grille cloth and trunk liner also.
    Stay away from pure cotton, as it can shrink and wrinkle.
  • Spray Adhesive: $10
    3M #90 is a good product - that's the heavy duty. They also have a standard strength, which should work fine, too (I think that's #88)
  • Acetone: $5 For cleanup

The grand total for all materials is approximately $150.


Besides the brushes and other light tools mentioned above, I also used the following hand and power tools:

Router w/various bits and circle jig (I made my own jig)
Drill w/various bits
Scissors/shears (NOT your mom's favorite pair - they will be ruined!)
Dremel rotary tool w/various bits.
Tape measure
Hot glue gun
Disposable rubber gloves



The tape is going to protect the car from the fiberglass resin. Using as much tape as you need, completely cover the area that will be molded. Use several layers of tape, until you are sure that your vehicle will be adequately protected. Use criss-cross layers, laying tape in opposite directions on each layer, and extend beyond the area to be fiberglassed. I used about 120 yards of tape to protect my vehicle.

It is also a good idea to draw a line in marker, to show the approximate dimensions of your finished enclosure. This will help you determine where to lay the fiberglass. My enclosure actually wound up much smaller than the marker line would suggest, but I used the line as an outside boundary I did not want to go past.


This was one of the toughest portions of my project, and at times, I considered quitting before I went any further! The fiberglass mat has a tendency to stick to everything except where you want it - the brush, your hands, etc.

Lay down your protective drop cloth, and put on gloves.

To start this layer, I first cut the fiberglass mat into manageable sections. DO NOT cut the mat like I did! Instead, TEAR it into smaller pieces. The frayed ends will help the matting stick where you want. I broke up one package of matting into various sizes, from about 4" to 12" pieces.

Next, I mixed the resin and hardener (catalyst) according to the manufacturer's instructions. Go ahead and mix up at least 16 oz, or maybe more - depending on the size of the area you plan to cover. Adding more catalyst will speed up the curing time, and reduce the working time. Using less catalyst will increase the cure and work time. I found that using just what the can says is probably best (imagine that!).

First, I "painted" some resin onto the tape, and tried to stick the mat pieces on to it. This worked eventually, but was still a bit of a struggle, as the mat had a tendency to pull away from the tape and fall off. I believe tearing the mat, and using a bristle brush would have helped me in this stage.

Once you have a piece of matting stuck, proceed to brush on more resin. Do not use a true "brushing" motion like paint, more a "stippling" motion - tapping the resin into the mat. You will see the mat start to soak up the resin, and once it is transparent, it is saturated.

Continue working like this, sticking on more mat and soaking it until your entire area is covered with the sticky mess. Once the mat is soaked, you should be able to once again see your marker line you made before, assuming you covered it (I did not). I did not glass up to my marker line, as you can see. However, you may or may not go past yours. It's up to you and the size you would like your finished project to be.

This is one stage where I wasted a lot of resin. Because I used foam brushes, used scissors to cut the mat, and it wasn't sticking very well, I really slopped the resin on there to get things to stay in place.

It is recommended that for subwoofer enclosures, the fiberglass be at least 7 layers thick. This sounds like a lot, but you will find that things speed up greatly once you get the first layer done, and your confidence increases, too. Try to lay as many layers as you possibly can right now. It is better to do more layers at once than to do one, let it cure, then continue. And once you start to build up the mold, sticking on more mat becomes very easy, as it adheres almost instantly. If you can do 5-7 layers right now, you're in great shape.

Now go grab a beer or two - you've got 2-4 hours until this mold cures.


Is it solid yet? I was really excited when I finally went out to my car (just to check on things) and found that the sloppy, sticky mess had become rock hard! It made all the frustrating work really worthwhile.

This is another stage where I realized my mistakes. First, I forgot to use a "mold release agent". See, once the fiberglass is cured, it will be firmly stuck in place. Common practice is to use mold release wax (I have read that Vaseline or a spray with WD-40 works fine) on the tape to help the mold pop out after curing. Well, I forgot. AND - I think it would have made the already arduous process of getting that matting to stick to the car even more difficult. Regardless, this is why I recommended the use of the blue painter's tape. Since my mold was stuck to the tape, and the tape was stuck to the car, it was quite difficult to remove. Using the blue tape will help the mold release with the tape still attached to it, which is fine. Using the standard masking tape like I did made things rather hard. I don't feel that the mold release agent is totally necessary, in retrospect. After all, I DID get the mold out.

Regardless, it should take some firm tugging to pop the mold. Be careful not to break the fiberglass. Another good reason to lay up as much as you can during the initial layering; it makes the mold that much stronger, thus there is less chance of cracking it when trying to pull it from the car. Keep working around the piece until you get it removed. After lots of pulling, mine came out with a loud "bang!".

Once the mold has been "popped", you can determine what the final back shape should be. Clean up the tape from the car, and re-insert the mold. Fits like a glove, huh? Nice! Using a marker, draw a line on the mold where you feel you'd like the final enclosure to be. This need not follow the line you originally drew on the tape. Create a nice shape that will work for you and your vehicle.

You can now take the mold to a better workspace for trimming.

To trim the mold, I used a dremel rotary tool with a small diamond cutoff wheel. This worked great, and is much easier, smoother, and maybe faster than a jigsaw. The pros will use air saws and the like - but we are home-builders, not pros! I found the dremel to be a great way to trim the mold to my line.

A quick note on what is to come: Upon removing my mold from the car, I first put in the sub rings, the construction of which is detailed in the next section. I would definitely suggest trimming the mold first. In the section that shows the mounting of the subwoofer rings, you will see that my mold is "ugly" and the edges are ragged and untrimmed. Hopefully, yours will be cleaner and neater, because you will trim the mold before proceeding! If you are unsure of the positioning of your subs, it might be better to wait on the trimming like I did. For a larger enclosure where the space is not so tight, and the subwoofer position has already been determined, trimming it right after the curing is a good idea.


I actually made my rings before doing any fiberglass. They were done in preparation for the rest of the project. You can make them whenever you want, however.

To make the rings, the first thing I did was to sandwich the 1/2" and 3/4" MDF together making a 1 1/4" thick piece. Wood glue works just fine for this - simply brush on a nice layer of glue (be sure to get it everywhere for best adhesion), clamp the pieces together, and let dry. Overnight is best, and I just put a TV on top of the sandwich as a "clamp"

Using the manufacturer's specs, determine the size of your ring. If you are using a 10" subwoofer, for example, you will want a ring about 12" in diameter. This will give 1" extra around the woofer. You can go larger if you want, but smaller than 1" might be tough later on.

The first step is to attach the circle jig to the router. I said earlier that I made my own - all I did was replace the standard router base with a 24" long, 4" wide piece of 1/8" thick balsa wood, with the router at one end. To make circles, simply measure from the bit and drill a hole in the balsa. Screw the router assembly to the MDF through this hole, and when you rotate the router around the screw, you get a perfect circle.

It is important to cut the largest part of the ring first - if you cut out the center first, for example, you will have nowhere to screw your circle jig to anymore. Start from the outermost cut, and work inward.

After the first cut, I now had a "disk" that was 12" in diameter, and 1 1/4" thick.

My next step was to rout out the "flange" that the sub would eventually mount to. Since I was using a 10" subwoofer, the outer edge of this groove (also called a rabbet) needed to be 10" in diameter. I used a larger router bit for this, to make a nice wide rabbet, and it was milled approximately 1/2" deep.

The cutout diameter for this subwoofer is approximately 9 1/4" the cutout is the hole that the sub needs to fit through. The last step was to cut out the middle, leaving a 9 1/4" hole.

As a final "dressing" to the ring, I used a round-over bit to "ease" the outside edge of the ring, removing the sharp corner and leaving a nice soft radius.

**IMPORTANT NOTE about using the router:

When making measurements for your circles, it is important to remember which "side" of the circle you want to keep. For example: When cutting out the "disk", you need to measure from the INSIDE of the router bit to determine where to screw through your circle jig. I used a 1/4" straight cutting bit for this. Now, had I measured from the OUTSIDE of the bit, the outside of the circle that was cut would have been 12", but the disk itself would have been 1/2" less in diameter, and useless. Similarly, when I made the rabbet and the cutout hole, I needed the OUTSIDE measurement to be the accurate one. Use your head, and remember what part of the bit to measure from. Much of this can be avoided by buying a commercially-available circle jig, on which the measurements are already pre-marked and indexed. But you can go the cheapo route like me if you think before you rout.


Here's where your sub enclosure is going to start to take shape. Determining the final position of the sub(s) might be tough or easy, depending on your application. Mine was quite tough. The fact is, I thought I was going to have more room than what I wound up with. I originally planned to have the subs facing the same direction. However, due to space constraints, it was necessary for me to angle the subs to get them both to fit, and finding the proper angles for the space I had was difficult. A single sub setup, or a larger enclosure, would help in positioning the woofers without interference.

Using the wood dowels, I began cutting pieces and attaching them to the rings and to the back of the enclosure. Basically, the idea is simply to support the rings in any way possible, so long as they are reasonably sturdy, and the supports do not interfere with the woofers or magnets. I used a hot glue gun to secure the dowels.

I have seen setups where the supports were made with scraps of MDF, steel strapping material - basically anything. Use whatever you feel is best, and whatever works for you.

Remember - I had NOT yet trimmed my mold. Under most circumstances, you would probably have yours trimmed before reaching this phase.

After the sub placement was determined, and the rings were reasonably secure, you will want to add more fiberglass to the mold. This will not only add more rigidity, but you should attempt to "embed" the dowels in fiberglass. This will permanently set them in place, and allow them to brace the final enclosure. If your mold is nice and thick already, just add fiberglass to the dowel attachment points. If you have any weak spots in the mold, now is the time to reinforce them. Don't worry if you go over your nice cleanly trimmed edge - you can always re-trim it later.

NOTE: Fiberglass is stronger in curves than in flat areas! Any areas of your enclosure that are gently curving or flat should be reinforced with extra layers of fiberglass. You can also insert rope, or small strips of wood, dowels, MDF, or almost any other material underneath several layers of fiberglass to artificially create "ribs" to act as curves and strengthen the enclosure. In the first picture, the upper left corner of my mold was relatively flat, and somewhat weak. Not only did I add extra layers to this area, but I also cut it back significantly when I trimmed the mold to shape. Any large areas of your mold that can be flexed with hand pressure should be well reinforced.

We're getting there....


Okay - now it's finally time to make this thing look like...well...something!

For the front of the enclosure, I used a polyester/cotton blend material from a fabric store. The elasticity is very important - you want to be able to stretch the material enough to remove any wrinkles and make the surface nice and taut.

I cut a piece of material (which I'll call "fleece" from now on) large enough to cover the front of the enclosure with some excess on all sides. Using the 3M spray adhesive, I started at one of the subwoofer rings. To use the adhesive, first spray both surfaces to be bonded together. You then let this sit for a minute or so. Once the adhesive is barely tacky to the touch, it's ready to bond.

After tacking the fleece neatly to a small area of one of the subwoofer rings with the adhesive, I began stretching it over the rest of the enclosure and sticking it down, working in small areas at a time. Try to pre-stretch the material, so you know what direction to pull to avoid wrinkles once the adhesive is sprayed on.

It is important to note that the fleece should be pulled around the back of the mold, and the adhesive used to stick it down. Do not simply use the adhesive on the edges of the mold, and think that it is sufficient. It will stick for now, but once the resin hits it in the next step, it will release. If too much of your adhesive gets released by the resin, the fabric can pull off the enclosure or wrinkle. Be sure to tack it down well away from the edges, where it will not be affected by the resin.

Once the fleece is stretched and tacked, the enclosure really begins taking shape, and you can start to see the results of all your hard work.

Now get out the resin again, and soak the fleece well. Be sure to saturate the fleece thoroughly where it contacts the MDF rings, and at the contact points with the original mold. It's not necessary to resin the areas where the sub holes will be cut. Once the resin cures, it will be permanently bonded to the back and rings. Depending on what material you use as your "fleece", this step can take quite a bit of resin. The thicker materials will soak it up like a sponge. This is fine - just make certain that all of the contact points are soaked well, and the rest of the taut fleece is well saturated with resin.

The fleece alone is nowhere near strong enough to be used as the final enclosure, so more fiberglass needs to be added for strength. My original plan was to cut out the subwoofer openings, and add fiberglass to the inside of the fleece, trying to maintain a nice smooth exterior surface.

This did not work out very well!

Using the router with a flush-cutting bit (straight cutting bit with a bearing that follows the sub ring shape for a flush cut), I removed the fleece from the sub holes. I then attempted to lay fiberglass inside - this proved to be nearly impossible, as there is so little room to work inside, and the dowels were win the way of nearly every move I made. I did manage to get a few sloppy layers done, but it took a very long time, and I wasted a lot of resin in the process. Do yourself a favor - unless you are making a very large enclosure for an 18" subwoofer (big hole!), forget about doing it this way. Just add fiberglass to the outside of the front.

Once I gave up on the inside work, glassing the outside was a joy. It was easy, quick, and came out fairly smooth, as I was careful to be as neat as possible.

Between the 2 sloppy layers inside, and these 5-7 outside layers, the enclosure became very strong once the resin cured. I have since been able to stand on it without consequence - I weigh about 155 pounds. Fiberglass is great stuff!

After curing, I again used the dremel to carefully trim off the overhanging fiberglass, and used the router to re-trim the sub holes.


The options for finishing your enclosure are almost limitless, however the most popular finish materials are carpet, fabric, vinyl, and paint.

Carpet and fabric are probably the easiest to accomplish. They are forgiving of slightly rough areas, and create a durable, good looking finish in a few quick steps.

Vinyl is a bit tougher, as it does not always stretch as easily over curves, and has a tendency to show most imperfections.

Paint is probably the most labor-intensive finish to apply. Just like painting a car, if the surface is not totally smooth, any tiny imperfection will show through the finish.

In order to apply paint (and in many cases, vinyl, also), a layer of body filler must be applied over the fiberglass front, followed by hours of laborious sanding and re-layering of body filler to obtain a perfectly smooth surface.

Is it any wonder why I chose a fabric covering?

Applying the fabric was simple. I first lightly sanded any rough spots in the fiberglass surface, including feeling for little "nubs" that would create bumps in the fabric. Be careful, as sanding fiberglass will make you itch for days, especially if done aggressively. I sanded as little as possible. The surface need not be perfectly smooth, but noticeable bumps should be taken care of. Then it was an easy process of using the spray adhesive to attach the fabric to the enclosure, in a similar fashion to how the fleece was stretched to form the front earlier.

For my enclosure, I used two layers of the fabric, because it was rather thin. Some of the small imperfections in the surface were showing through, and the second layer helped to disguise them.


It would be a shame to go through all of this work creating a nice enclosure, only to mount the subs with inferior hardware. Screws in MDF are adequate at best, so the use of something stronger and more durable is definitely recommended. I chose the popular method of using "tee nuts" to fasten the subs to the rings.

To use the tee nuts, you drill a hole in the MDF, and insert the tee nut in the hole, inside the enclosure on the bottom of the sub ring. Then a corresponding bolt is threaded through the sub mounting hole, through the MDF, and into the tee nut. The tee nut has "teeth" that will bite into the MDF and provide a very secure mounting once the bolt is tightened down.

A note about tee nuts: If you know the orientation of your sub(s) BEFORE mounting the rings to the mold, go ahead and drill the mounting holes and insert the tee nuts at that point. Use a hammer to set the teeth into the MDF by banging the nut into place. Unfortunately, I was unsure of the exact position of my subs before the enclosure was built, so I had to install the tee nuts afterwards. This proved to be a hassle, as the only good way to set the nuts in place is with a hammer - and obviously, a hammer did not fit or swing well inside the finished sub box. It was necessary for me to pre-tighten a bolt into each of the tee nuts to set the teeth into the MDF, and even this did not work as well as hammering them in place. Several of mine fell out later on. If at all possible, hammer them in before mounting your sub rings in the mold.

MISCELLANEOUS STUFF (but still kinda important)

Here I will detail a few specific things that were not covered in the rest of this tutorial.

First will be wiring - where do the speaker wires come into the enclosure?

To accomplish this, I used a common terminal cup. I first cut a piece of MDF to secure the cup to. This particular cup has a circular cutout, and a square flange, so my piece of MDF was cut to match. Then, using the MDF as a template, I cut out a portion of the back of the enclosure, in a space I knew would work in my application. I used the hot glue to tack the MDF into the hole, and then fiberglassed the MDF in place during the reinforcement stage of the mold.

This picture shows the MDF glassed in place. The red circle is a hole cut in the mold, which lines up with the jack mount in the side of my car. All it takes is a bolt through this hole and into the jack mount to secure the enclosure to the car, along with a rubber washer to keep the enclosure airtight.


I always knew I wanted a "different" looking grille. It also had to be sturdy and protective, as my hatchback carries various tools and boxes that can occasionally get knocked around. I had the idea for this grille in mind from the beginning, and it is one reason the subs were lined up the way they were - I paid careful attention to where the mounting holes were, in order to place them inline with each other, so the grill would cover both subs in at once. This alignment is another reason I was forced to place my tee nuts in at that late stage of construction.

To make the grille, I used 1/2" thick solid aluminum rod - purchased from Home Depot. Aluminum is fairly easy to bend, and I actually used two pipes I had laying around the house for leverage. I inserted one pipe over each end of the rod, stood on top of one, and pulled up on the other to bend the aluminum evenly. After some trial and error, I came up with the correct angle for my application. I then carefully bent the other two rods to match, lined them up on the enclosure, and drilled the mounting holes.

The rods actually are an integral part of the project, as they share mounting points with the subwoofers. They are also "propped up" on small pieces of copper pipe (painted black) so as not to interfere with or touch the subwoofer surround during play. So each of the screws goes through the aluminum rod, through the small copper pipe piece, through the sub, through the MDF sub ring, and into the tee nut. The grille must be installed along with the subs. It's not as complicated as it sounds, I suppose - the grille simply shares it's screws with the subwoofers themselves.

And that's about all there is. This was a fun project - difficult at times, especially for a first-timer like me, but very rewarding. I am very pleased with the final outcome. I have since hooked this setup to a 600 watt amplifier, and I am very pleased with the sound - it is as good or better than I had hoped, which is quite satisfying!

Hopefully this tutorial will assist others with their own projects. Free distribution of this write-up is encouraged. Any commercial use is strictly prohibited. Either give this information for free, or don't distribute it at all. Selling of this document, or any other for-profit use, in part or in whole, without the express written consent of the author is completely prohibited.

If you have any questions or comments, feel free to email me at [email protected] - especially if this tutorial was helpful to you. I'd love to see what you've done!

I would also like to thank any and all who helped me to get this project completed - your knowledge and experience was well received and appreciated. Here I have done my part to "give something back".

QUICK NOTE: "Fleece is better to use than mat because of its strength. You can use any type of material as long as it can be soaked in resin. Fleece is easier to sand and fill too. In order to get it smooth you need to first sand the material. Then use as little of body filler as you can. Then fill the real tiny holes/ scratches with feather fill. then sand with 400 then 800 grit sand paper.
This is all in my glassing experence"

Now get out there, and get messy!

Courtesy of 355Nation Members:

Codered said:
A good supplier of quality rollers, matting/cloth/ and resin is

A good learning site is

some other finishing materials are

Duraglas (fiberglass reinforced filler) and Rage Gold (premium body filler thats easy to sand and 100 times better than bondo)
courtesy of

Premium Member
21,130 Posts
Re: How-To: Fiberglassing for Noobs

not bad man

mind if I add a couple good fiberglassing links?

Premium Member
21,130 Posts

1,035 Posts
Re: How-To: Fiberglassing for Noobs

Sweet after seeing this my GF won't see me for days im going to fill the whole Can HEII ya thanks.
Best right up i have seen in a long time :bow2:
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