One-Off Construction Using
Fiberglass Over Foam
An Overview of How to Work with Fiberglass Over Foam
by Robert Q. Riley

The automotive application of FRP/foam composite was pioneered in the 1970's by the author's former design firm, Quincy-Lynn Enterprises, Inc.  We used polyester resin and glass cloth laminations over a core of rigid urethane foam boardstock (fiberglass over foam).  Modern variations sometimes use an epoxy resin or vinyl-ester resin and carbon-fiber-cloth over urethane foam, like the composite used to build GM's Ultralite experimental passenger car.  Carbon fiber materials are far more costly, and it's questionable as to whether the value matches the extra expense.  Despite the strength-to-weight advantage of newer materials, nothing matches the original polyester-resin/glass-cloth/urethane-foam composite for quickly producing a tough, lightweight product at rock-bottom costs.  And it's a surprisingly simple system to use.

You can use this composite to make virtually anything, from a vehicle body to a boat.  The following description takes you step-by-step through a fiberglass/foam composite buildup of a vehicle body. To illustrate the process, we've included photos taken at various stages during the construction of Tri-Magnum; a high-performance three-wheeler for which plans are available on this site.  The techniques, however, will work equally well in other applications.  And Tri-Magnum plans include far more detail than is shown in this overview. 

If you want to go ahead with a custom project (not necessarily Tri-Magnum) and just want to know how to do it, the XR3 Hybrid plans-on-disc provides the greatest detail, plus an hour-long video showing how the fiberglass is applied and reinforced.  (See the box at the bottom of this page.)   If you're planning a project in fiberglass/foam composite, the XR3 plans-on-disc will give you a much deeper understanding for minimal expense.  Plans also cover hinge, fixture, and glazing design, which lets you design your own hinges and fixtures and correctly anchor them into the fiberglass.  This composite process also works equally well for building a plug for a traditional fiberglass mold. 

A Low-Cost and Forgiving Material

A typical automobile body will require about $900 worth of plywood, urethane foam, and standard polyester fiberglass (the same material used to build boats, kit cars, and production Corvettes).  The body is first built of inexpensive foam boardstock (typically, 4 x 8 foot panels of 1-inch thick material) cemented in place over station formers.  The foam core or buck can also serve as a low-cost, full-size styling mockup.  Figure about $250 for enough plywood and foam, and about three or four days of assembly time to build a typical automobile body-in-foam. 

If the design does not work well in three dimensions, it can be modified by stacking on extra foam and reshaping it.  Or you can discard the first attempt and start over with only minimal loss in materials cost.  And assembly is virtually painless. Using a razor knife, urethane foam cuts like butter, and it sands so easily you can rub two pieces together and watch it melt away.  Fiberglass is applied to encapsulate the foam only after the design has been fully refined.  Smoothing out the fiberglass to prepare it for paint is the most labor-intensive part of the process.  But even here, modern plastic body-fillers and spray-on primer/surfacers like "Featherfil" and "Eliminator" make the job far easier than the original sanded-gelcoat method used when the system was first developed.

A body project begins with the chassis supported at ride-height.  Normally, the wheels must be  removed so they do not interfere with the smooth curvature of the foam panels.  Plywood station formers are then attached at appropriate locations along the length.  The correct placement and number of station formers will depend on the body design and the location of convenient attachment points on the chassis. Station formers normally become part of the finished body where they serve as cabin bulkheads, a firewall, and/or body-to-chassis attachment points.   Click photos on along right of page for a large version. 

Some builders may prefer to calculate the shape of foam panels from drawings. A quicker, hands-on method is to attach temporary stringers across station formers at each break in the shape (to become foam panel mating points). Body panels can then be traced directly onto the foam using the stringers as a guide. Cut foam panels oversize then shape them to fit as they are installed. Use a solvent-based contact cement to bond the foam to adjacent panels (water-based cements do not work well on urethane foam). It's best to wear a particle respirator when working with exposed urethane foam (before it is fiberglassed over) to avoid inhaling airborne particles caused by sanding.  At commercial sites, these dust respirators are required by OSHA.

The correct sequence of assembly will depend upon your particular design. Tri-Magnum's foam body begins with lengthwise panels installed at the widest point of the body. With other designs, it may be better to install the floorboard first and work upward. Where interior structures are essential for strength, install them and secure the seams with a lay-up of fiberglass mat on the inside. With Tri-Magnum, most of the passenger compartment was assembled and fiberglassed before the roof panel was installed.  Often, however, interior details are easier to install after the body has been fiberglassed.  The idea is to build a bodyshell and stabilize it with fiberglass before tackling all the details.  The correct construction sequence depends on structural considerations and on which items will be inaccessible later on.

The foam body begins to look more like a finished product as details are added. The one-inch-thick foam boardstock provides plenty of material for deeply rounding the corners to eliminate the panelized look. Urethane foam is easily shaped with a Surform file or a sanding block. Some of the details may require strengthening with a lay-up of fiberglass as they are installed. For example, Tri-Magnum's wheel well eyebrows were extremely thin and they extended quite far from the body. Consequently, they were reinforced with fiberglass so they would hold their shape and stand up to the occasional assault of a dropped tool.

Outline doors and windows with a felt marker then sand recesses and cut-lines into the foam using a sanding block.  Windows are left intact to avoid releasing tension that might be holding a curvature into adjacent panels.  The intricate detailing possible with urethane foam is apparent in the following photographs.

Apply a Fiberglass Skin to Encapsulate the Foam

Lay-ups of fiberglass transform the fragile foam body into a real-world automobile body. Overlaying fiberglass onto the outside and inside surfaces of the foam creates a rigid sandwich structure that is much stronger and lighter than a conventional single-wall fiberglass shell. A typical exterior lay-up consists of two layers of 6 ounce fiberglass cloth and polyester laminating resin, applied one lay-up at a time.  In order to avoid uneven surfaces, do not use mat on the exterior.  Apply similar lay-ups (two cloth) of fiberglass over the back side to completely encapsulate the foam. Extra cloth lay-ups and mat reinforcement may be used on the back side, along inside corners, and over areas where the foam has been sanded especially thin. When fiberglassing is done, cut hatches, doors, and windows from the body, then detail the interior.

Add Returns, Gutters, and Attachment fixtures After Fiberglassing the Exterior

It is possible to progress to this stage in little more than three or four weekends. Now, however, progress will seem to slow to a crawl as details such as nut-plates, flanges, rain gutters, and other interior elements are built into the body.

Smooth the Surface and Apply Paint

The process of leveling and smoothing a fiberglass body is the most labor-intensive part of construction. Literature on fiberglassing describes various techniques for producing a gelcoat finish. At Quincy-Lynn, however, we used conventional autobody repair techniques and filler materials instead. The fiberglass body is first smoothed with a Surform file to remove large fiberglass drips and surface irregularities. It is then covered with lightweight plastic body filler (such as "Bondo") and leveled using traditional body-working techniques.

Catalyze a small amount of body filler then apply and smooth it with a plastic squeegee. Cover about two square feet at a time. Wait until just after the filler sets, then level the surface with a Surform file before going on the adjoining area. The small, half-round files work best. Do not wait until the material has completely hardened. Use the Surform file when the body filler is still soft enough to let the file bite easily into the filler, producing small, curled strips of material. If the filler granulates or rubs off (instead of cutting cleanly) it is not yet hard enough. If the body file skips across the surface without biting into the material, the body filler has become too hard. Once fully hardened, the surface will have to be leveled with a sander, which is a much slower and more labor-intensive process. Immediately after the surface has been leveled with the file (before the material fully cures), use a sanding block with 36 grit paper to remove the file marks.  After the filler has cured, smooth out the surface with 100 - 120 grit sandpaper.

The leveled body is then sprayed with a polyester primer/surfacer, such as "Featherfil" or "Eliminator." It can then be painted with any of the traditional automotive grade paints. A polyester primered finish is not quite as hard as a traditional gelcoat finish.  However, results come much more quickly.