Fiberglass Molding - Topping It Off - Fabricating A Multi-Composite Enclosure Part 4

Properly mixed body fillers should have approximately 10 minutes of working time before beginning to set. If this is your first project using body fillers or a specific brand, I recommend mixing a small test batch, without applying it to the project, and use a stopwatch to see how long it takes to set. This will give you a reference for how much hardener should be added to the filler. Different types and brands of filler require different amounts of hardener for proper working times. Some reinforced fillers will not fully cure if sufficient cream hardener is not added, while too much hardener will not give you enough working time. Be sure to thoroughly and completely mix the hardener with the filler (for at least 30 seconds) to avoid having soft or uncured areas. Creating a smooth shape with body fillers takes time and practice. Applying a smooth coat can help reduce grinding and make things easier, but is not critical at this stage. Cutting the plastic spreaders to custom shapes can help ease spreading. The spreaders can be cleaned with acetone and reused.

What exactly is "fiberglass-shaping material"? Although this is subject to much debate, it can be a variety of different materials, as long as it's stretchable and absorbs fiberglass resin. There is no exact science to picking a material, and it doesn't need to be a particular blend of fibers. I've read false claims on Internet forums that the fiberglass-shaping material must be a 50/50 blend of polyester and cotton. As long as there aren't any chemicals in the material that prevent the resin from curing (I haven't seen one yet), the exact blend of fibers shouldn't make any difference. Popular materials are speaker grille cloth, t-shirt fabric, fleece, stretchable speaker box carpet and "custom mold fabrics." I don't recommend using thicker materials like the speaker box carpet because they soak up too much fiberglass resin and tend to warp. Fiberglass resin and "shaping material" without fiberglass mat (or other strengthening material) is brittle and prone to cracking. Using a fuzzy material should also be avoided because it can create a coarse texture when the resin cures, making it necessary to sand the entire surface smooth before reinforcing it with fiberglass mat. I highly recommend using a light colored speaker grille cloth (which just happens to be readily available in most car audio shops) or a thin, smooth material similar to it.

Although I usually apply the reinforced filler and allow it to cure overnight without touching it, those of you without access to compressed air or air tools will want to take one more step before calling it a day. Instead of allowing the filler to become rock hard before sanding, sit there and wait for it to reach the "green stage." Don't call your girlfriend, don't have a beer and don't go to the bathroom (if you must go, go really quickly!), because you might miss your golden opportunity to save a lot of sanding time. After the filler sets and completes its exothermic reaction (in which it heats up and starts cooling back down), it reaches a stage when it can be easily sanded using 36-grit sandpaper. If the filler "rolls" off with the sandpaper, wait a little longer until it sands off like grated Parmesan cheese. Taking the time to knock down the high spots, spreader marks and rough areas during this "green stage" will save a lot of time and effort later. Next time we will cover the finishing stages of our composite enclosure.

Tools and Supplies:

Once the ring is completely fastened, pull the shaping material at the outer edges of the frame and begin stapling around the perimeter. The shaping material should be taut, but does not need to be as tight as a drum. Stretching too tightly can cause the material to tear. Pull and staple the perimeter in sections from opposing sides so that the material is stretched evenly. Concave sections of the frame that don't make contact with the shaping material need to be fastened with additional staples, but convex sections that already make contact don't. Once everything is securely fastened, carefully cut off the excess shaping material with a sharp razor blade. Take care not to drag the blade across the staples as it will dull it immediately.

Although the fiberglass shell is not fully cured until it sits overnight, we will be applying a fiberglass-reinforced filler (e.g., Duraglas/Everglass) to smooth out the shape at this stage so that the resin and filler can both cure overnight. The purpose of applying the reinforced filler is to fill in low spots and get the top of our enclosure closer to the desired shape. Some areas may need quite a bit of filler, while other areas will need very little. Fiberglass-reinforced filler rather than standard body fillers is used for the first stage of shaping because it is stronger and less prone to shrinking and warping, especially when filling areas over 1/4" thick. To prepare the fiberglass top for the filler, use 36- or 50-grit sandpaper to knock down any rough spots and wipe the surface clean of any dust. Thoroughly mix a batch of reinforced filler with the proper amount of cream hardener and apply to the entire surface.

Automotive Upholstery Store:Light colored speaker grille clothDuoFast TI-3118A pneumatic upholstery staple gun w/ 3/8" crown x 3/16" or 5/32" 22-gauge wire staples

Home Improvement Store/McMaster.com:36- and 50-grit sandpaper

As the application R&D supervisor at Alpine Electronics from 1995 to 2003, Chris Yato was in charge of creating and managing numerous demo vehicles that served as centerpieces at CES. All of them have been featured inthis magazine. He is currently the special projects manager/fabrication trainer for Competition Soundworks in California. E-mail him at chris@teamcsw.com.