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How To Fiberglass a Custom Sub Box

30K views 52 replies 19 participants last post by  SGPMan 
#1 · (Edited)
I had originally ordered an ST1 because I was going to replace the stock radio anyways with better components but I found out that the only face plate available from Metra looked horrible and made the car look cheap. I decided to purchase an ST2 but upgrade all the speakers and subwoofer while using the stock radio. I'm actually happy I got the ST2 b/c I really like Ford's My Touch. The speech recognition is just unbelievable when I tell it an address to route to.

Anyways, as for the stereo, it does sound decent but the major weakness is the sub. Its not clean sounding and ends up distorting the mids and tweeters. There's not a lot of room in our trunk so I decided to use a shallow mount sub to mount in the corner to replace the stock sub. I'm not a fan of putting big sub boxes in the trunk b/c it's not aesthetically pleasing and takes up too much usable space. I fiberglassed a sub box in my last car so decided to do one for my ST.

First, in order to remove the stock sub, you would need to remove the spare tire and then unclip the wire that attaches to the sub. It's attached to the green connector in this picture and you just press the tab to unlock it and pull it out.

Photography Metal Silver


Remove the foam that sits next to the spare tire and you'll see the 3 bolts that hold the sub in place:

Metal



This is what the sub looks like removed from it's enclosure:

Audio equipment Loudspeaker Auto part Technology Electronic device


I plan to replace it with an Alpine SWR-T12, it only requires a 0.6 cubic foot sub box and is 3.5 inches deep:

Subwoofer Car subwoofer Loudspeaker Vehicle audio Audio equipment
Auto part Camera lens Lens Machine



Things you will need:

Fiberglass resin
Fiberglass mat or cloth (I find the cloth is easier to work with)
disposable plastic cups
disposable gloves
Ventilator
painters tape
petroleum jelly
acetone
cheap brushes


First thing you'll need to do is tape the area you want to fiberglass. It's very important that you double tape everything because the resin will stick to anything and once it's on your carpet/plastic trim or car then it'll be there permanently. I then outlined the area where I want the sub box to be with a marker.

Green Hood Plastic


Apply a generous amount of petroleum jelly to the area, it will serve as a releasing agent to help you remove the fiberglass frame later.

Green Automotive exterior Hood Trunk Vehicle


Now cut up the fiberglass mats into smaller pieces, about 4x4 inches. Make sure you wear gloves and use a ventilator mask when working with fiberglass.

Sand Paper


Mix up a SMALL amount of the resin in a plastic cup. In a red solo cup, I usually pour to the first line at the bottom and add about 20-25 drops of the hardening agent and mix it really well with a plastic spoon. Use the brush to dab a small amount onto the painter's tape and then lay a fiberglass mat on it then dab some more until you soak the mat thoroughly. It'll turn from white to clear once it's soaked thru. Move from one area to the next, using only a small amount of resin at a time because you only have about 15 minutes to work with it before it hardens. Don't mix too much at a time or else you'll just waste it. In between mixing the resin, be sure you place the brush in a glass of acetone to prevent the fiberglass resin from hardening it. Here's what it looks like after 2 layers:

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Add about 4 layers of resin & mat and l let it dry overnight. I used a screwdriver to carefully pry at the corners and loosen all the outside edges.

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Leaf Plant


Once all the edges are pried apart, yank on the bottom (holding onto foam insert) and you can hear it slowly releasing. This is the most difficult part because it's stuck on pretty good but if you pull on the corners, you'll eventually get it freed.

Vehicle door Trunk Vehicle Automotive exterior Auto part
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Clean the back side of the fiberglass frame with some dish washing soap to clean off the petroleum jelly. After cleaning it, I put it back into the car to mark where I need to trim it.

Food Dish Cuisine Ingredient Comfort food


Using a dremel, I cut it to the size I want and file away the rough edges. Here's the final back side of my sub box.

Automotive exterior Vehicle Auto part Car Trunk
 
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#2 · (Edited)
The next step is to attach the sub ring that I had cut out of an MDF board. I used hot glue to attach the dow rods and then glued it to the back of the sub box.

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Now take an old T-shirt and stretch across the front of the sub box and use the hot glue to secure it down.

Paper Plaster
Paper Plastic bag Paper product Art


I then used a marker to outline where the border of the sub box is

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Soak the T-shirt thoroughly with resin and then layer it with fiberglass mat

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I'll let it dry overnight and then cut out the center and continue layering the resin & mat from the inside.
 
#3 · (Edited)
Here's the box with the center cut out and after some rough sanding:





Test fitting the sub into the box:

Audio equipment Car subwoofer Subwoofer Lens cap Loudspeaker
Rock Grass


Making sure the box fit into it's corner:

Auto part
Land vehicle Vehicle Car Trunk Auto part
Automotive exterior Auto part Bumper Fender


It's a tight fit, but I'll make it fit. I'll just need to file away some of the foam inserts.

I then applied some Bondo and then sand it down with 40 grit sandpaper to even it out as much as I can:

Font Concrete Circle


I bought some black car carpet from Autozone and used Loctite spray on adhesive to glue the carpet to the box.

Tire Automotive tire Spray Material property Gas


I'm not very good at carpeting, there's some seams that I'd rather not be present but this is the best I could do:

Land vehicle Vehicle Car Trunk Mode of transport
Vehicle Car Trunk Audio equipment Subwoofer


And with the top cover on, it's hidden from prying eyes:

Land vehicle Vehicle Car Trunk Auto part
 
#52 · (Edited)
looks like just what I was thinking!

Here's the box with the center cut out and after some rough sanding:


View attachment 7222 View attachment 7223


Test fitting the sub into the box:

View attachment 7224 View attachment 7225

Making sure the box fit into it's corner:

View attachment 7226 View attachment 7227 View attachment 7228

It's a tight fit, but I'll make it fit. I'll just need to file away some of the foam inserts.

I then applied some Bondo and then sand it down with 40 grit sandpaper to even it out as much as I can:

View attachment 7229

I bought some black car carpet from Autozone and used Loctite spray on adhesive to glue the carpet to the box.

View attachment 7331

I'm not very good at carpeting, there's some seams that I'd rather not be present but this is the best I could do:

View attachment 7332 View attachment 7333

And with the top cover on, it's hidden from prying eyes:

View attachment 7334
The finished product looks amazing and is exactly what I am thinking of doing for my Focus ST. I am planning on pretty much following your instructions to the "T" as I have never done fiberglass fabrication before. I imagine the project is complete now so how do you like your custom sub? would you have done anything differently if you were going to do this project over again? What did you do to get the wires into the enclosure and maintain a tight seal? how did you mount it to the car?

I am really impressed with your end result! I plan on doing this project within the next couple of weeks or maybe in November so I would really appreciate any suggestions you have!

Also, did you encounter any issues with My Ford Touch afterwords or any issues with steering wheel or dash electronics?

Thanks
 
#9 ·
No, the stock amp won't be able to power a decent aftermarket sub. My setup will consist of:

Dynaudio Esotec components up front
DLS coaxials in the rear
Alpine sub
JL HD 600/4
JL HD 900/5
Bit One processor

I'm still debating how to connect the stock radio to the Bit One. I've read some posts where I'm told to wire it before the stock amp while others say the signal is too low so I would wire it after the stock amp. I'd prefer to wire it before since the amp may distort the signal.

Electronics Technology Electronic device Room Guitar amplifier
 
#11 ·
Wow that looks pricey! What is your total cost so far?

Where are you going to mount the amplifiers? To the back of the back seats?

Most of the stuff were taken from my other car.. Only the speakers and sub are new, so it wasn't too bad. If you invest in quality stuff originally then you can use them in your next car and not worry about wasting them.

The processor will go over the stock amp and the 2 amps will be placed right next to the stock amp. I will have to cut into the fender foam to make them fit but I don't like the look of amps screwed to the back of the seats.
 
#15 ·
I really enjoy watching projects like this evolve. I've only ever done a little bit of fiberglassing in my life (college project) and man did I hate it. :) I've never done much with audio systems in my cars - always having preferred go-fast parts. But really am liking your project so far.

Side question - I remember itching like crazy after working with fiberglass (particular after sanding). What's the trick?
 
#19 ·
The Alpine sub needs 0.68 cubic feet of space, which is approximately 5 gallons. I took this picture and asked my brother if he thinks the box is big enough.

Food Dish Ingredient Cuisine Junk food


He didn't think it would hold 5 gallons and I questioned my initial measurements. There's only one way to find out, here's a 5 gallon bucket filled with water:

Soil Concrete Bucket Pipe Drain



And the result:

Drainage


Sorry, bro, you owe me a steak dinner :yay:
 
#28 ·
Finally finished my box. How does it sound? I really don't know.. part 2 of my stereo install is wiring up the amps. lol

I know.. I should have wired first and then made the box, but the power wires didn't arrive in time for me to start this project when I was free one weekend so instead of waiting around, I started to build my box instead.
 
#32 ·
That looks incredible! Very clean. Unfortunately, I've given up on car audio. I've found anytime you start taking things apart, they rattle when you put them back together. That, and powerful bass seems to shake every panel in the car loose. Maybe I'm just not doing something right...lol. Either way, it seems like you have very good taste, can't wait to see how the whole system turns out while using the MFT still.
 
#33 ·
It's all about the prepping. Lots of people focus on the major stuff, like replacing the speakers and amp and forget about the sound deadening aspect of the install. If you take the time to address these areas then you shouldn't have any problems with rattles. My last car was a Honda Fit and the entire interior was made of cheap plastic. I sound deadened the whole car and after I was done, the sound of the doors closing was similar to a Mercedes, very solid and quiet. It's all about the technique and attention to detail when it comes to car stereo install. It's not very expensive to get the sound deadner, but it makes all the difference in the world in the install.
 
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