Unless you own a vintage Chris-Craft or a restored steel trawler, you probably have a boat made of fiberglass that could use improvements and repairs here and there. While fiberglass work may seem daunting to a novice do-it-yourselfer – all those heavy chemicals and resins – it does not have to be.
Common polyester resin and fiberglass rolls are available off-the-shelf at just about every marine store in the country, and learning to work with these products will give you the freedom and flexibility to upgrade your boat in ways other materials cannot match.
With some basic skills and practice, you can make major cosmetic improvements, replace worn and broken parts and build entirely new structures. Electronics cases, deck boxes, access hatches and vents are all good candidates for a first-time effort.
To illustrate the process, I'll use one of my own small projects: replacing the lid of a top-loading sailboat refrigerator that was too thin and poorly insulated. My goal was to reinforce part of the existing lid with fiberglass, add foam insulation and bond the whole thing to a teak frame. It would cost me a few hours of work, but the result would be a vastly improved fridge for less than $100 in materials.
START WITH A MOLD
Modern fiberglass applications have evolved a great deal. The development of plastics reinforced with various fabrics began during World War II, spurred by a lack of aircraft aluminum and other shortages. L. Francis Herreshoff, cantankerous son of the famed designer Nat Herreshoff, called it "frozen snot." But the material grew to dominate the industry.
Fiberglass, glass reinforced plastic (GRP) and fiberglass reinforced plastic (FRP) are all basically the same thing: fibers of glass suspended in a matrix of resin. For our purposes here, that resin will be polyester. There are more exotic materials out there, but for many boat applications, good old polyester resin still works just fine. The glass fibers give the matrix its strength while the resin adds stiffness, a classic example of whole adding up to more than the sum of its parts.
Paul EsterleMost fiberglass projects start with a mold.
In much the same way that your boat was built, most fiberglass projects begin with a mold – male or female – that is used to create the structure you are building. Female molds are hollow and the project is formed inside them. With a male mold, you form the project over the exterior.
Most fiberglass boats are built in a female mold. The first step in making a hull is to thoroughly wax the mold. This wax develops the shiny surface of the hull and provides the mold release so the hull can be removed. A gel coat, much thicker than paint, is then sprayed into the mold and allowed to cure. Then the fiberglass reinforcement and resin is added.
The process is similar for small projects: build the mold, coat it with mold release, apply the gel coat and then add layers of fiber and resin.
Because we are building solitary parts, rather than manufacturing, the mold need not be complicated. I have made molds from cardboard constructions and blocks of wood glued together and sanded to shape. In most cases you won't get the shiny surface you get from a permanent mold, but you won't have spent the time, money and energy required to make such a mold either. Another strategy is to make the mold from thin plywood and leave the plywood sealed in resin as part of the finished product.
RELEASE IT
With my refrigerator, I got lucky and was able to use the original lid as a starting point, so I didn't need to build a mold from scratch. The heart of the lid was a shallow rectangular pan with one sloping side, which matched the opening in the countertop and allowed the lid to open. I used the pan as both the outer surface and the mold for the fiberglass reinforcement.
Paul EsterleMolds need not be fancy. The author built his from an existing refrigerator lid.
I cut the top and four sides to the exact size required and then beveled the inside edges so the pieces would mate well. I then taped the pieces together to form the pan that acted as the mold for my fiberglass.
If you plan on removing parts from a mold, some sort of mold release will be required. One option is PVS (polyvinyl alcohol), which can be brushed or sprayed on the surface. Heavy applications of wax also will work, but be sure to use a Carnauba-based wax and not a silicone-based product. I have even used melted candle wax as a mold release on small projects.
The next step is usually to brush on a layer of gel coat. Use a laminating type of gel coat that will remain tacky, and aim for a uniform thickness with no bubbles. The mold, together with the tacky gel coat, forms the foundation for the fiberglass reinforcements and resin.
WORKING WITH RESINS
Fiberglass is applied in layers to minimize heat build-up associated with the curing resin.
Polyester resin is available in two basic flavors: laminating and finishing. It is laminating resin that is used to build up the layers. Laminating resin is "air inhibited," meaning it will not cure tack-free when exposed to air. This is a good thing when building up layers of resin and reinforcement, because each succeeding layer bonds both chemically and structurally to the one below it.
However, it also means that polyester resin and its complements are best suited for new construction. Applying new polyester laminations over old severely limits the chemical process, leaving only a structural bond – and even that depends on proper surface preparation. For projects that incorporate existing fiberglass structures, epoxies are often a better option. Polyester resin requires a small amount of catalyst, which triggers the polymerization that will turn the resin from a vicious liquid into a solid material. The amount of catalyst needed depends to a great degree on the ambient temperature of the room. Most resin containers have a chart indicating the proper amount of catalyst that should be used in relation to the ambient temperature.
Using more catalyst than recommended will shorten the "pot life," or working time, before the chemical reaction starts. As it begins to polymerize, the polyester resin will heat and that heat helps cure the resin faster. Large pots of resin with the catalyst added will cure, or "kick," faster so you should begin by working with small batches. Keeping the resin cooler will prolong the pot life, and it is much safer. Large pots of resin can actually "boil over," melt plastic mixing containers or catch on fire. Consider yourself warned.
The catalyst is also very dangerous, especially if you get it in your eyes. Always wear eye protection, as well as proper gloves. Polyester resin also smells really bad and has high levels of volatile organic compounds. Many boat builders have gone away from open laminating and moved to resin infusion or resin injection molding to comply with federal emission standards, those these techniques are more advanced and require expensive equipment.
The goal is to be cautious and develop good habits. Follow the manufacturer's instructions, check your measurements several times before mixing, work outdoors or in well-ventilated area and be aware of fire dangers that may be posed by fumes and by overheating the resin mixture.
FIBERS OF GLASS
Of course, resin is only half of the equation. The other component is the glass fiber reinforcement. Modern fiberglass design includes a wide range of exotic reinforcements such as Kevlar and carbon fiber, but I'll stick to the common materials that are widely available and reasonably priced.
Paul EsterleAbove is fiberglass mat. Below is fiberglass cloth (inside) and roving (outside).
Glass fibers are exactly that: molten glass spun into fine fibers that are then processed into different types of reinforcement. The first type is fiberglass mat. Strands of fiberglass are cut into short lengths and randomly compacted into a felt-like material. The strands of glass are held in place by means of a glue or sizing applied to the mat.
Chemicals in the resin dissolve the sizing, allowing the mat to conform to whatever it is applied to. The short length of the glass fibers makes it weaker than other types of reinforcement, but it adds thickness and stiffness to a molded part more quickly than other options.
Woven roving is another common type of fiberglass reinforcement. This material is like cloth, but made with extremely thick yarns. It is much stronger than mat, but working with it requires more resin and more effort. Typically, this course material is used in combination with fiberglass mat, which helps fill in the roving's rough surface and eliminates the possibility of air bubbles in the matrix.
Fiberglass cloth is also available in a variety of sizes and weights. It is usually measured in ounces-per-yard, as in "9-ounce-per-yard" cloth. It is available in pre-cut packs, as well as tape and rolls, with widths ranging from 1 to 50 inches. Weights can range from fractions of an ounce to nine ounces or more per yard. The cloth is stronger than mat and conforms to surfaces better than woven roving. A common use of fiberglass cloth is sheathing plywood boat hulls. The fiberglass eliminates problems with the plywood "checking" or cracking, and improves both strength and stiffness. Cloth is often used in moldings where higher tensile strength is needed.
THE APPLICATION
This first application will usually be a layer of mat. After cutting the mat to size, lay it in the mold and brush on a coat of resin. Use cheap, throwaway brushes because cleaning them will be nearly impossible. Work the resin into the mat until the mat turns translucent.
Make sure you work out any air bubbles from each layer. This can best be done with a grooved fiberglass roller. These rollers force air bubbles to the surface, and insure good contact with the layer below. After the initial layer, you can add more mat, roving or cloth to build the project up to its final thickness.
Pay attention to the resin-to-reinforcement ratio. Ideally, this should be 50 percent resin and 50 percent reinforcement. On a practical level, that ratio is nearly impossible to achieve without more advanced techniques, but make every effort to contain your enthusiasm for applying resin. Use just enough to completely wet the reinforcement and insure good contact. Often, you'll end up using more resin than is ideal, but try to minimize overuse as best you can.
Paul EsterleThe author created a jig to keep the project steady and to shape while he glassed.
On my refrigerator lid, I started by applying a small amount of fiberglass filler to the inside corners of the pan (I did not use gel coat). I smoothed these out with the end of a popsicle stick and immediately applied a coat of resin, followed by a layer of two-inch wide fiberglass tape. This stiffened up the pan and sealed up the edges.
The upper edges of the pan were a little floppy and bowed. To keep things straight, I clamped pieces of plywood to each side and placed each side of the pan down on a piece of waxed paper as I glassed the sides. I was more interested in stiffness than looks, so I built up each side and bottom with several layers of mat, rolled with my grooved roller. This resulted in a stiff, strong pan that I then bonded to the teak frame of the lid.
FINISH WORK
Keep in mind that projects involving a temporary mold will not give you a finished surface, and you'll have some work ahead to complete your project and make it pretty.
You will need to fill and sand, then apply a final finish in order to make the surface tack free. One option is finishing resin, which has a wax material added to the mix. The wax rises to the surface and seals it off from contact with the air, allowing the surface of the resin to cure.
You can also apply paint or gel coat. My choice is often to use paint, because the thick gel coat is hard to apply to an exterior surface with a brush. One-part marine polyurethane paints, such as Interlux Brightsides or Pettit EasyPoxy are cost effective and easy to touch up. Two-part paints, such as Awlgrip or Interlux Perfection are tougher coatings, but also more expensive and difficult to touch up.
Paul EsterleThe resulting product was bonded to a teak frame. It made a far more efficient refrigerator.
Gel coat is available in most marine stores and comes in finishing and laminating types. Most marine gel coats are white. Kits with a white base and tubes of color are available, but color matching gel coat is an art and often usually less than perfect. Even if you make a match, the old and the new coats age at different rates and may evolve into different colors over time.
As for my new refrigerator lid, the laminate that made up my mold became part of project, giving me a smooth white surface and sparing me the process of exterior finishing. In the end, the new lid held almost four inches of polyurethane foam and was covered by a teak plywood top plate, a construction that dramatically increased efficiency.
Paul Esterle is Technical Editor for Small Craft Advisor Magazine, and his freelance work has appeared in Sail, BoatWorks, Voyaging and Good Old Boat magazines. He has produced a series of boating videos and lectures widely. He also works at West Marine and has written product reviews for the company.