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Building a fiberglass enclosure for a sub unit is a simple and effective way to free up boot space and improve sound. You can easily make your box completely sealed and positioned using the sides or spare wheel well. Creating the exact box volume, which is critical for some woofers, can also be done using this method.

You will need to clear the boot area as best as possible, remove any loose debris and vacuum.

Here is the problem. Big ugly box with amps in the back. No boot space. The plan is to fibreglass the two 10" (250mm) subs into the side walls.

Once everything is removed from the boot you can make the MDF front baffles. To follow the contour of the car use several pieces of cardboard taped together as templates. Use the templates to cut out MDF baffles and install to test fit. Remove baffles and cut mounting holes for the speakers making sure they are symetrical. The panels will be bonded to the fibreglass later.

Wipe surfaces to remove dirt or other contaminates and ensure everything is dry. Carpeting should be removed to avoid getting any resin on it, although it could be left in as you will be covering all surfaces anyway.

Tape up the entire area you want to make the box in with masking tape plus at least 6" (150mm) over. It is much easier to remove excess than to add more later when you discover it is too small.

Use as much tape as you want. it is critical that there are no gaps, holes or leaks as the resin will want to stick itself to the inside of your boot if it Its allowed to get through. Cover the masking tape with resin proof tape to stop the fibreglass from sticking. Aluminium foil and release wax can also be used as a release.

Now cover the rest of the boot with medium polythene, you will be glad you did when you get splashes, drips or spills of resin. It always happens.

Now look at the shape of what you've done, once its fibreglassed does it look like its going to get wedged in? Are there any undercuts or returns that will prevent the removal of the moulding?

The best fiberglass matting to use for this job is 450g chopped strand mat.

Its light enough to be easily worked and heavy enough to give a reasonable thickness of laminate. You will eventually need enough for a minimum of 3 layers to give you a solid moulding, so work out your total area now. Multiply the total area (in square metres) of all three layers by 0.45. This will give you the total weight of mat to get. Allow plenty of excess to allow for overlaps, and some wastage.

Use standard general purpose polyester resin.

You need to multiply your total weight of mat by 2.5 to get the approximate amount of resin in kilos you'll need. It is only an approximation, some people use a lot more than others.

Unroll plenty of fiberglass mat to cover the area. Rip some of it up into smaller pieces, for tight corners, which will need patching in.

Now mix up some resin and hardener. The trick here is not to mix up too much as you have a limited time to use it up. No more than one litre at a time, especially if you are inexperienced using resins. Check the air temperature if you can, if its over about 20°C then cut back on the catalyst to 1% to give a longer working time. Curing rates are affected dramatically by temperature, the same applies to cold conditions so don't start if its below 5°C as this may stop the resin curing altogether. For the 5-20°C range 2% catalyst should be ok. Obviously move your car into the shade, out of direct sunlight.

Paint the surfaces with resin using a brush then place a manageable piece of fiberglass mat directly onto the wet resin and push it on. Big creases should be ripped while the mat is still dry. More resin is then applied to the back, don't paint it on, stipple it in, and go for it, the mat soaks up a surprising amount of resin. After a few minutes the binder in the mat will break down, and you can now work the fibreglass (called consolidation) with a small paddle roller or brush. You need to get as much air out as you can because air bubbles will weaken the laminate. Also the mat fibres will spread out leaving a smooth surface with no gaps between fibres. Any area that has not gone transparent and is still white in colour is usually the result of insufficient resin addition. More resin must be applied to these areas and allowed to soak in and then re-consolidated.

Ideally, you are looking for one even layer all over, but don't worry about overlaps, its better to have two layers in an area than to have missed it out altogether. Leave to set solid.

Now is the time to remove the moulding. It can be left in place, but the more work you do inside the car, the greater the chance you will spill or splash resin on something you wish you hadn't.

Add another 2-3 layers in the same fashion as before. Leave to set solid,

You should test fit it now, just to make sure it goes back in. Trim the edges using a jigsaw with a blade for plastic cutting. Use sand paper to remove any sharp edges and rough spiky areas.

You can now fit the MDF front baffle. Look at trying to bond this on permanently with more fibreglass to try and prevent leaks. Ensure any MDF is heavily scratched to get a good mechanical bond and all external corners are rounded off. The base can be screwed into place then use wooden blocks for internal bracing to reduce vibration.

Around the edges are small and large gaps. The small gaps are easily filled by applying a few layers of glass then trimmed.


Larger gaps can be filled initially with MDF then fibreglass. After all gaps are filled you will end up with a sealed box.


Fit speakers and equipment. Cover with carpet and staple down.

Check list.
450g chopped strand matting,
general purpose polyester resin,
masking tape,
resin proof tape or release wax,
resin brushes,
consolidation roller,
mixing buckets,
catalyst dispenser or Syringe,

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