Custom fiberglass without a mold

Sometimes it's desireable to build something out of fiberglass, without the use of a mold. This is often true in car audio, where you are building a single structure, such as a dashboard, or a speaker box.

Many articles cover building a speakerbox into a stange cove, by fiberglassing into a corner or hole, but what they don't show you how to do, is to build a completely custom object, such as a speakerbox with a curving exterior shape.

There are a few things to decide up front. First, do you want to use fiberglass as the primary structural component, is the fiberglass what you wish to use to hold the majority of the strength? Do you want to simply use the fiberglass to build certain areas that are impossible to build with MDF? Or do you want a fiberglass "piano finish" as your end result?

First, some notes on fiberglass. It will not form clean right angles, no matter how hard you try. Second, if you sand down into the actual fiberglass mat or cloth, it will look like crap. Third, it is very difficult to work with over wildly curving surfaces.

Now, it's important to think about these factors when trying to build something with fiberglass. You need to think about the end result, not how to get there. If you want your structural base to be entirely fiberglass, you have allready made your decision. If you simply want curving lines, then you shouldn't use fiberglass at all. If you want a piano finish, you shouldn't use any mat.

Lets start with the last one, the piano finish. This is usually the goal of most fiberglass work. You can easily paint something to achieve this look, but it requires a bit of skill with a paintgun, and it doesn't give off the deep look a fiberglass finish will, unless you are extremely good with paint. For a piano finish, what you really want to do, is only use the resin. Get some finishing resin, preferrably the stuff used to make surfboards, ultra high gloss. Paint it on to a perfectly smooth surface. This is key, any imperfections in the surface will be magnified by the fiberglass. Mix the fiberglass with a coloring agent, and use alot, to give a very opaque finish. Paint it on thick, and cover everything evenly. If you need to, do a second coat before the first coat loses it's stickyness. I suggest between 3-5 coats. On the final coat, thin the resin with acetone, this will help fill in some of the brush lines.

After curing completely (2-3 days) begin sanding. Progressively sand with increasinjg grains of sandpaper. Start out with a high grain, like 400, and sand the whole surface. As you sand, it will cut down the raised areas, leaving the pits and valleys a very high gloss, making them easily visible. If possible, sand these raised areas down to the lowest points in the pits. You may have to use a coarser paper to achieve this. When sanding, allways attempt to use a random orbital sander, the scratch patterns of other sanders, or hand sanding are not desireable. If you are sanding a cruved surface, use a foam backing pad for your sander, to allow it to cover the curved areas. You can get foam backing pads up to 1" in thickness. This will allow you to do some pretty wild curves.

Once you have the whole area sanded to a consistent dull look and smoothness, you should then work your way back up to the 400 grit paper. Purchase some "Abrasol" pads for your sander. These are foam pads available in grits from 180 to 4000, specifically made for creating ultra-fine finishes. They work best when used with water. Clean the surface of your box with a tack cloth to get all the dust off. Then, using a squirt bottle set on mist, completely soak the box. Use the abrasol pad on your random orbit sander, making sure the pad is allways rotating at a high rate of speed. If it slows down, squirt more water on the area. Constantly sand and squirt. The 180 pad should be used to start (following the 400 grit paper). This will leave a shine to the fiberglass. You should inspect carefully for deep scratches, and work them out with the abrasol pad if at all possible. The abrasol is more of a polisher than a sander, it can remove scratches, but will not cut down into material. When sanding with abrasol, lubrication is the key, so use plenty of water.

Keep working upwards through the pads, constantly checking for scratches from the previous pad. By the time you get up to the 2000 and 4000 grit pads, you should be able to see your own reflection in the fiberglass. Once you have smoothed it all out with the 4000 grit pad, it's time for a trip to the auto store. Pick up some car polish, preferrably the type designed for use with an orbital grinder. Get a polishing pad for your grinder, and polish like mad. Then apply a hand polish per the instructions on the bottle. At this point, your finish should be scratch-free, and mirror-like. If you like, wax this finish, like a car.

Now what if you want to create these wild shapes? First, you need to know what the end result will look like. It helps alot if you have the ability to visualize things like this in your head. If not, perhaps you can draw it, or have someone draw it for you. Then, you need a base to begin construction on. For example, say you wanted to add a few curves to your dashboard. Starting from scratch would be difficult, but it's easy to add on. Start by taking the dashboard, and determining how much material you need to add. Anywhere where you will be raising the surface by more than about 1/2", you should build it up with foam. Get construction grade polyurethane two-part foam. This is available in a spray box, which contains two cans. Do NOT use expanding foam available from the hardware store unless you absolutely have to. Expanding foam leaves huge bubbles in the inside of the foam. Spray the foam over the areas you wish to raise, add more foam than you plan to raise the structure. Allow it to dry, and then shape with a file/kitchen knife/sandpaper. Shape it out to the desired design, and give it a coat of paint. Bondo will eat away at foam, so you are creating a barrier between the foam and bondo. Add about 2-3 coats of paint, to make sure all the micro-holes are filled in.

Now, apply bondo liberally. Shaping as you go with the bondo. With bondo, it's best to work with small amounts at a time, as it cures really fast. Build up thin layers, shaping them with a spatula to get the desired shape and flow of the end product. Once you have built your shape, use a cheeze-grater (a file, that looks remarkably like a cheeze-grater, not an actual grater) to shape the bondo to the desired shape. It's best to work with the bondo while it's hard, but not fully cured, as it will literally work like hard cheeze, allowing you to tkae off alot of material very quickly. Allow it to dry completely, and then sand it down with a 40-60 grit to shape.

Once you have the shape you want, now get a cheap glossy paint. Doesn't matter what color, but black or red work well. Paint the surface. Now, just like in the instructions for the mirror-finish above, you want to sand it down, looking for pits and valleys which will be highlighted by the glossy paint. If you have some pits that are extra deep, you can build them up with a few layers of paint, or add some more bondo. Continue to sand, or alternate painting and sanding, until you have a consistent dull finish. At this point you can do as you like with the box, paint it, vinyl it, or go back up and mirror-finish it.

Now if you want to build an entire structure, free-form from fiberglass, you are in for some fun. You essentially have two ways of doing this. 1) build the bottom, and then the top, or 2) build the whole thing in one go.

If you can build the bottom first, it's a bit easier. but the procedure is the same. Create a template for your fiberglass out of two-part polyurethane foam. Sand and carve this foam to the exact shape you want your item to be. Take great care not to leave any large pits or mis-shapen areas. The upper surface of the foam template will be conveyed through the fiberglass directly. Any deformities will be amplified in the finished product. If you need to, use bondo to reshape slightly.

Once you have the template finished, begin applying fiberglass mat over it. Depending on how strong you want the end structure to be, you will probably need quite a bit of glass. Long thin strips are easier to work with over large curving surfaces, where large pieces are better for flat areas. Use a laminating resin for all work.

Once you have finished building up the fiberglass, you will need to decide how you want to finish the product. If you want to do a piano finish, just start adding the layers of finishing resin right over the final layer of laminating resin. To paint the object, the procuedure will be the same, but you don't need to dye the resin. You will probably need to sand it out just like the other projects above. When using paint as your final coat however, you can use a brush-on paint to make smoothing go much faster. You can simply layer the paint on, and sand through it, which is much easier than sanding down resin to get it perfect.

Once that's done, you need to remove the template fromt he interior of the object. Manually remove as much as you can, scraping and cutting it away. You can use gasoline in tiny amounts to dissolve the foam, and wash it out.

Now you have a custom fiberglass box. Aren't you proud? Here are some links to help you out a bit:

Tools you will want to buy

I also recieved this nice letter from a fellow who used my instructions to build a custom enclosure for his car. His enclosure turned out great in my opinion.

Thanks a million for posting your tips on building fiberglass speaker enclosures out on the web. This was my first stab at it, but it turned out great. Attached are some pics. It's a sub enclosure that fits between the center console and rear seat of a Chevy Avalanche, the shape was necessary to keep rear AC and allow front seats to fully recline. I used a foam plug as the base and followed your instructions applied 7 layers of 1.5 chopped mat. Then got a tip from a friend to mix long-strand fiberglass filler with resin (about 40/60 filler to resin) and coat the interior of the box to ensure 100% seal and help build up additional thickness. Covered the enclosure with matching vinyl to give it a factory look.

He also included these pictures, click on them for a larger view:

Tim Rightnour
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Last modified: Tue Jul 6 12:34:48 MST 2004