Do you really need to use fiberglass?

Working with fiberglass is a very messy and time consuming process: Prepare area, lay fiberglass, wait for it to dry, sand/cut if necessary, lay more fiberglass, wait, sand, an so on. Once you are done with fiberglass, repeat the process with Bondo (car body filler) for finishing: Apply, wait, sand, reapply, wait, sand. It might take several days, even weeks to do a nice set of kick panels, subwoofer box or amplifier rack.

If possible, try to determine if you can use an alternate material such as wood and then shape using Bondo. Keep in mind that fiberglass is strong when bent. Straight fiberglass panels have to be very thick (read: time and money) for adequate rigidity. Sometimes a combination of wood, MDF or particleboard (for large flat sections) and fiberglass (for round, odd sections) works best.


  1. Fiberglass mat or cloth, resin and hardener.
  2. Bondo (body filler), hardener.
  3. Box of disposable gloves, respirator, protective clothing.
  4. Paint brush, plastic sheeting, aluminum foil, mold release or WD40.
  5. Tools such as sander, multi-purpose shears, screws, etc.

For small projects, such as small amplifier racks or small kick pods, you can buy all the supplies at a car parts store such as Trak Auto. Products can be found at the “body repair” aisle. For bigger projects, supplies can get pretty expensive. Boat supply stores sell products in larger quantities, but at lower overall prices.


Fumes and dust particles are a very important concern when working with fiberglass. Get a respirator or a dust mask designed to work with fiberglass. Wear gloves at all times when handling fiberglass and resin, or sanding. Protect ALL exposed skin, especially when sanding.

Work in an open area! Resin/hardener mixture fumes are bad for your health. If you work with resin indoors, the smell will remain in the area for days. Do not handle fiberglass mat or sand dry fiberglass indoors. It causes rashes and itching.

Read instructions and warning labels carefully.

Car Preparation

Before any work starts with fiberglass, plan the whole project. Look ahead into how you are mounting speakers, components, fastening the panels, panel finish, etc.

Once resin falls on carpet, upholstery, or other parts in your car, there is no way to get it out. Cover areas to be worked thoroughly. If possible, remove panels, seats, carpeting, etc in case an accident does occur.

Cover area to be molded with fiberglass with aluminum foil. Fiberglass can be laid over the foil and once it dries, foil can be easily peeled off.

Making a Mold

If you are creating a shape in “mid air”, you need to make a mold first. There are different options available. Some people like to make a frame out of aluminum foil and/or chicken wire. Other people use modeling clay or shape dried spray expanding foam.

Another option is to make a “skeleton”, shape it with cloth and then fiberglass over it: Make a top and bottom part out of fiberglass, wood, plastic, existing car panels, etc. Join both with wood or metal braces. To fill the gaps, glue or staple sweatshirt material or pantyhose. Apply resin to the cloth or pantyhose. Once they dry, lay fiberglass over it.

The third option is to use an existing shape, such as a spare tire hole in a trunk. After removing factory panels and carpeting, apply mold release, aluminum foil or WD-40 to surface (to avoid fiberglass from sticking). Lay fiberglass, and let dry.

First Layer

First, mix resin and hardener. Only mix what you will need. It takes a lot of practice to get the resin/hardener ratio right. Too much hardener and it will dry right away, too little and it could take several hours. Temperature in work area also influences drying time. The hotter the temperature is, the quicker resin will dry. Also, keep in mind that resin will get warn when drying.

Cut fiberglass mat to size. It is better to cut a bigger size than what you need for the first layer. You can always trim excess off when dry.

There are two ways to “wet” the fiberglass mat: By dipping it in the resin/hardener mixture, or by applying resin with a brush. In most cases, it is easier to dip the mat.

Once you have a wet mat on your hands, place it on the area that will “shape it”. If you are a beginner, this might be a bit tricky. The mat will tend to stick to gloves and other stuff you don’t want it to. Spraying some WD-40 on your gloves will help a bit solving this problem. This first layer would become the foundation of the piece you are building.

Additional Layers

Once the first layer is dry, remove it from the car. In most cases there is no need to work inside the car for subsequent layers.

Cut and sand excess fiberglass from fist layer, clean dust. Add next layers in same fashion as fist layer. Try not to have any gaps or bubbles between layers. You can use a cheap 1″ brush to help get rid of bubbles. Do not worry about imperfections at this stage, you just want a rough shape with no major protrusions. All gaps and imperfections will be fixed at the last stage.

Shape and use of the object will determine amount of layers required. For kick panels, 3 to 4 layers is usually enough. Subwoofers boxes require more layers.

Bondo Stage

Once you have a defined shape, no major holes and a pretty sturdy piece, you need to smooth out by sanding rough edges.

Bondo is very similar to fiberglass. Just add a few drops of hardener and drying process begins. Spread Bondo over you panel. Try to fill in gaps and valleys. Do not worry about smoothing it out much. Once Bondo dries, sand. Repeat the process as many times as necessary: Add Bondo, let dry, sand.

On the first steps, a power sander can be used to quickly remove excess material. On finishing stages, manual sanding might be required, depending on finish desired.


Finish smoothness depends on what material you are using to cover the piece up. Carpet is very forgiving when it comes to imperfections. Vinyl is less forgiving, you need a pretty smooth surface (a couple extra steps of Bondo might be required). If you are finishing with paint, then you do need a perfectly smooth surface.