The Basics Of Fiberglass Fabrication - How To

Cut 0.25" outside the marker line with a jigsaw using a metal cutting blade. It can be very difficult to securely hold the shell while trimming the edges and fiberglass particles will fly through the air. Use all of your protective gear and take extreme caution.

Prep the area where the frame and fiberglass meet with a coat of resin on the edge of the MDF, as body fillers sometimes don't bond well to raw MDF. Mix a batch of reinforced filler with the right amount of hardener and apply it to the MDF. Place your frame on the shell and use a plastic body filler or gloved finger to spread the filler around the inner and outer perimeter. The filler should harden in 15 to 20 minutes, after which grind down the outer edge with a 24-grit Roloc disk and block-sand it flush.

You can buy plastic body filler spreaders in various sizes, and you can also trim them in custom shapes. Either wipe them clean or leave them soaking in a bucket of acetone.

For efficiency, prep your sandpaper beforehand. Use 3M Super 77 spray glue to stick sandpaper back to back or make custom sanding blocks. Use cheap scissors to cut sandpaper to size and thick Safegrip gloves to protect your hands from the rough paper.

Now make the speaker mounting ring out of 11/42" MDF (31/44" MDF for subwoofer rings) using a Perfect Circle router jig, which I'll attach to my Porter Cable 690 plunge router with a 0.25" solid carbide spiral bit. You can round over the inner edge of the top ring or use a 45-degree chamfer router bit.

Use a rabbeting bit to notch the outer edges of the fence and mounting ring. This provides maximum contact area and strength for the top of the enclosure as well as a guide for the staples we'll soon attach. It also lets us build the top flush without grinding away the strength of the MDF and fiberglass joint. The width and depth of the notch depend on how thick you want the fiberglass top to be. For a single sub enclosure, a 0.25" wide by 0.25" deep rabbet should be sufficient, while larger enclosures require a larger notch. Be careful as rabbeting bits are dangerous. A rabbet should be cut in 11/48" increments for safety. You can also cut a rough notch with a die grinder, Dremel tool or chisel/file/rasp. Knock down the sharp edges of the MDF with a Roloc grinding disk.

Make at least four stilts from 31/44" MDF and attach them to the fiberglass, frame and ring with commercial-grade superglue and an aerosol accelerator. For a nice shape, use a jigsaw or band saw to cut out 11/42" MDF ribs. Using a cardboard pattern will make the process easier. Make sure each rib meets flat with joining surface and superglue them securely.

Now it's time to stretch fiberglass-shaping material over your ribs. Speaker grille cloth, jersey, fleece, stretchable speaker box carpet and custom mold fabric are popular. I don't recommend thicker materials, such as speaker box carpet, because it soaks up too much resin and tends to warp. Fuzzy materials can also create a coarse texture. I like to use grille cloth. Drape a piece over the top of the enclosure.