How to Build a Fibreglass Sub Box




Introduction: How to Build a Fibreglass Sub Box

This is the very first time i have tried a project like this - i had never used Fibreglass before so this entire process was a massive learning curve. I made a few mistakes which contributed to the overall time spent on finishing the project but the end result was well worth the time and effort invested in it.

If any of you are thinking in trying this at home i say go ahead and do it - just make sure you have a plan in place and try have all the right materials on hand to get the job done!!

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Step 1: Decide on the Area to Build in

Mask off the area you want to fibreglass - this will give you the back / mould of your sub box. Make sure you use proper masking tape (blue painters tape is a good option) - i used some cheap tape i had at home and the fibreglass resin made it extremely difficult to remove afterwards - session 1 learned!!

As can be seen on the pics - i put some screws into the wall of the box to secure the box with afterwards. You can do it this way or you can drill it afterwards but that might an issue with space in the completed box.

Step 2: Apply Fibreglass

Apply your fibreglass over the taped out area. I marked off the exact outlines for the box to give me a better idea where i needed to apply the fibreglass. Apply a few layers until you have a solid base - leave to dry.

Step 3: Remove the Mould From the Car

Once the fibreglass mould has dried you can remove it from the car and trim the edges. In order to trim the FG you will need a dremel or an angle grinder as i used.

Step 4: Speaker Ring

This step took a little time to get right... i prepared the speaker ring by cutting 2 circles out of a piece of wood and gluing them together, i then sanded them down to give me the finish i was looking for. Make sure you make these rings big enough to fit your sub in easily - mine was a little small so i had lots of sanding to do to make it fit - lesson no.2 learned.

I fitted the speaker ring to the sub mould using some wooden dials - i ended up drilling a hole through the mould to mount them because i didn't have a glue gun on hand. PS - make sure you test fit your sub to make sure you have enough clearance before moving on!! I fibreglassed the dials in once they where sitting in the desired location...

Step 5: Cover the Box With Material

You can now drape your material over the box and secure it down using contact adhesive / glue gun!! The type of material used is important - as can be seen from the photo i used a rather course material - this proved a mistake later on when i was trying to smooth the box for paint... lesson 3 learned...

Make sure you pull the material over as tight as possible - you don't want any creases in it...

Once the material is secured you can apply one or two coats of FG resin to the material and let it dry!!

Step 6: Apply Fibreglass

Once the resin cloth has dried you can apply some additional FG mat to strengthen it up - i would advise putting these layers on the inside of the box instead of the outside - once again this will make the finishing of the box easier... another lesson learned on my side.

Step 7: Body Filler

You now need to start smoothing the box out - apply Body Filler and sand down until you have a perfectly smooth finish!! When painting a box like i intended on doing that finish needs to be absolutely perfect otherwise all those little imperfections will be visible once the paint has gone on!!

This took ages but lots of the work can be minimized by following the correct steps in the building phase - more lessons learned for me!!

Step 8: Sound Deadening

I then sprayed the inside / back and bottom of the box with sound deadening spraypaint - i also applied some sound deadening matt inside the box once complete!!

Step 9: Primer and Paint

A couple coats of twin pack primer followed by the basecoat and clear and the project was nearly done!!

Step 10: Final Fitting

To finish the box off i ran the wiring and sealed the hole up with silicon - i then filled the box with polyfill to maximize the bass produced and fitted it back into the car.

This entire project was a massive learning curve for me but i am extremely happy i gave it a go and am ecstatic about how the box turned out and sounds!!

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    5 Discussions


    3 years ago

    Very nice build! I have wanted to make a curved FG box for a while now. What cloth would you think works best, knowing what you know??


    Reply 3 years ago

    Hey man - you need to use a smooth fleece material.... something that you will be able to stretch over all the curves... the material i used was a thicker fleece and as mentioned caused some issues when it came to smoothing the box out for paint!!


    3 years ago

    the fabric trick is awesome for creating complex curvature! i wanna use this trick in future project! thx!


    3 years ago

    Wow! That turned out looking so nice. Way to persevere and see it through to the end. It looks professional to me!


    Reply 3 years ago

    Thanks mate!! yip in the end it all worked out really well - it just took much longer than it could have!! But thats how you learn i suppose!! :)