The Basics Of Fiberglass Fabrication - How To

After double-checking that you have all the materials you need, preferably on a workstation covered with plastic sheeting, you can begin. Polyester laminating resin is a sensible choice. It should have the consistency of maple syrup and provide 20 to 30 minutes of working time at 70 degrees before it begins to gel. It should harden within three hours and cure within 24. For this type of project I only recommend fiberglass mat, which has equal strength in all directions and can be used in multiple layers. Different thicknesses can be used for different projects, but a good choice for this type is 0.75 or 1-ounce mat. It should be cut in 2" to 6" strips and shredded into square or rectangular pieces with frayed edges, which allow for better layering. Some areas will require triangular or arc-shaped pieces. Be sure to prepare all the necessary mat before applying the resin. Also have some acetone handy in a plastic quart cup.

Experience will teach you how much resin to mix per batch. For a medium-sized project try starting with 16 ounces. Mixing the proper ratio of resin and catalyst (MEKP or methyl ethyl ketone peroxide) is important as it provides consistent working times and cure rates. Five milliliters of the hardener is needed for every 16 ounces of resin at 70 degrees, reducing the amount by 50 percent at temperatures around 90 degrees and increasing it by 50 percent in temperatures near 50 degrees.

Stir the MEKP into the resin using a stir stick for at least 30 seconds for proper mixing. The mixture should change color. You should have 20 to 30 minutes of working time at 70 degrees. If it starts to gel before then, mix up a new batch, adjusting the amount of hardener by 15 to 25 percent (do not use resin that has begun to gel)

Brush a coat of resin within the guidelines to create a tacky surface.

Now place the shredded pieces of mat as cleanly as possible into a pattern that's similar to roofing shingles. This should provide a uniform thickness. On a vertical or inclined area start at the bottom working upward.

Quickly wet out the mat with the resin, using a bristle brush and a pressing/tapping motion. Work from one end to the other getting rid of all air bubbles under the mat.

When finished, the entire surface should have a transparent appearance without any wet/dry spots or puddles of resin. With exception of inverted surfaces, you will want the first layer to become dry to the touch before adding more layers. Add all layers in one sitting until the desired thickness is achieved. Large flat areas need to be thicker than curved or contoured areas, because they'll flex or resonate with pressure. For sub enclosures, 7.5 to 9 ounces or more of total mat thickness is a good idea with 4.5 to 6 ounces for cosmetic or smaller pods.

Let the shell dry overnight and cure fully. Don't use heat guns or lamps, as they can heat the surface unevenly. In cold climates, use an electric heater blowing into a large cardboard box or the trunk, which provides an even oven effect. Once cured, you may be able to pry the shape out with your hands. With larger shapes, you may want to use a plastic wedge to tap around the edges to pry it out. Use a pry bar with extreme caution and only with stubborn shells. Now you can also remove the aluminum foil with the help of compressed air.

For this project we'll use sides made of 11/42" MDF rather than fiberglass to save time. For a large sub enclosure 0.75" would be better. In our example, the base is level; if it weren't, we'd have to contour the bottom of the MDF. The bottom edge doesn't have to match perfectly to the shell, but any gaps should be less than half an inch. Use with glue and brad nails to assemble the sides and notch the top outer edge with a rabbeting bit.

Using a 2" 24-grit Roloc disk attached to a right-angle die grinder, rough up the bottom edge of the MDF fence. If you don't have this tool, used a coarse rasp or file.

Here's a closer look. This will increase the bonding area between the MDF in the fiberglass shell. Now, place the frame onto the base and trace its outline with a permanent marker.