Why might you want to work with Fiberglass?

Working with Fiberglass and resin is not terribly difficult, but it can be a messy proposition. The main reasons are to repair an existing fiberglass item, such as a boat hull, or to create something such as a custom spoiler for a car or truck, or to make a sculpture from fiberglass.


So, you ran your expensive fiberglass bass boat into the shallows, to chase some lurking largemouth bass, but your eyes and depth finder missed that rock lurking just under the surface of the lake. You lurch forward as the hull strikes the rock with a thud, but at least you weren't going too fast. After backing off of the rock, you realize that while you are not in any immediate danger of sinking, after a few casts in the immediate area, you realize that the bilge pump is running quite a bit more than it usually does. You start to get that sinking feeling as the realization hits that you did more than just scratch that expensive plastic because it is no longer able to seal the lake out of the boat.

You wisely decide to cut your fishing trip short and head back to the launching ramp before the leak gets any worse, and luckily, you make it back before sinking. Once out of the water, you survey the damage, a nasty gash and a six inch long area where the fiberglass is cracking. Back at the marina, you show the damage to the guy who fixes that kind of thing, and with the nearly toothless grin of someone recently transplanted from the land of Deliverance, he hands you an estimate equivalent to 3 monthly payments on the boat. You feel like Ned Beatty, but you are determined not to let this pirate with an air sander steal your hard-earned money, so you decide to try and fix it yourself.

Creating Custom Pieces

On the other hand, perhaps you have discovered that you have an artistic flair for sketching out custom ground effects and other bodywork for your nicely riced out Honda Prelude that really look good, but nobody makes what you like that you can just bolt on. You have even done some mockups in styrofoam, and want to take the next step and make something that can travel on your dream machine.

What both of you need to accomplish your mission is a bit of working knowledge about fiberglass. Hopefully, I can give you an idea about what it is all about, but if you are serious about working with fiberglass, I highly recommend that you read up on the subject from sources who are experts on the subject. The concept of working with fiberglass is pretty straightforward: You create a fiberglass surface by applying a resin such as polyester resin or epoxy resin to the surface, then apply alternating layers of fiberglass mat or fiberglass cloth and resin until you build it up to the desired thickness.

Fiberglass Repairs

I have sucessfully repaired the transom on a fiberglass boat using this technique, and have also used this technique to repair the desktops on some workstations I maintain at work. Repairing an existing object made of fiberglass laminate is easier than making one from scratch, so if your ambitions are to make your own fiberglass fenders or ground effects, you would do well to get some broken ones and experiment with them. Working with fiberglass cloth and resin is smelly, dusty, messy work, which will probably ruin whatever clothing you decide to wear. Polyester Resin uses a catalyst to polymerize the resin into a hard plastic, which is resistant to just about all solvents, including acetone.


Fiberglass Cloth and Mat

The fiberglass itself for doing repair or fabrication work comes in two main forms, cloth and mat. Fiberglass cloth is a coarsely woven fabric with about the same weight as burlap. Cloth is used where strength is important, but is less able to conform around irregular surfaces than mat is. Mat consists of randomly arranged short fibers pressed into a mat similar to the material in furnace filters (often they are actually made of fiberglass}. Mat is also cheaper than cloth, in terms of the weight of material put down. It is good for surfaces that need to be built up a bit, or to conform over irregular surfaces.

Polyester Resin

Polyester resin is the most common type of resin used in fiberglass work. It starts as a syrupy liquid which is mixed with a catalyst, which polymerizes the liquid resin into a solid material which is resistant to most solvents, including Acetone. Once the resin is mixed with the catalyst, you have about 10 to 20 minutes to work with it before it hardens, depending on the amount of catalyst used, and the temperature. Do not mix any more resin than you can use up in that time, and clean your brushes and other tools with acetone before the resin hardens, or else you will need to remove it with a belt sander. In its uncured state, polyester resin is flammable.

Epoxy Resin

Epoxy Resin is the same stuff as the two-part glue used to repair things around the house. It is not as brittle as polyester resin, so it is often used in high-strength applications in aviation, but is more expensive. Thankfully, it is cheaper by the ounce in the quantities sold for use for laminating fiberglass or carbon fiber than it is in the little tubes, but it is still more expensive than polyester resin. Like polyester, epoxy is messy to work with and is pretty much impervious to most solvents once hardened, so clean up your tools before it hardens or learn to live with epoxy encrusted tools.

Gel Coat

Gel Coat is simply polyester resin mixed with pigment to produce a solid color. It also has a surfacing agent, such as paraffin to produce a cured surface. It is the first coat applied when working with a mold, or the last coat applied when doing repair work. Without a surfacing agent, air tends to inhibit the surface from curing, making the surface sticky. Paraffin floats to the surface of the resin, and seals the surface from air.


Cleanup of cured polyester and epoxy resins is difficult at best. Acetone will soften the cured resins a bit, but will not remove them. Acetone is used for removing uncured or partially cured resin from tools, brushes, etc. It has a sweet smell, evaporates very quickly. It is also very flammable. Methylene Chloride is also used as a general solvent, but is also pretty nasty stuff. Styrene is used to thin polyester resin.

All the chemicals mentioned above can be toxic if inhaled, ingested, or absorbed through the skin, and some are also known carcinogens. Consult the MSDS sheets for more information on precautions and hazards associated with their use.

Other tools and supplies needed:

For making up mixed resin, you will need disposable containers to mix it up in. Shallow is better, since mixed resin generates a lot of heat, which can build up in a deep container. A disposable roller tray works well for this. Depending on the size of the repair, paint brushes or rollers with a short nap can be used to apply the resin. Other supplies include:

Nitrile gloves to protect your hands
Good quality dust masks, a must when sanding or grinding
Sanding or grinding equipment such as an angle grinder, disk sander, and sanding block.
A source of Compressed air is helpful for blowing away dust, or running air powered grinders or sanders
Polyester Resin based filler, such as Bondo
Paint or pigment to make up gel coat
Old Clothes: Something that covers well, and that you are prepared to throw away when you are done

Doing the Repair

After getting all of your tools and supplies together, it is necessary to prepare the surface to accept new laminate. In the case of the damaged hull, the area to be patched must also be dry, and any shattered glass removed. Rough up the area to be patched, removing any decorative gel coat about 4 to 6 inches beyond any damaged area to be repaired. In areas where the existing fiberglass is cracked or crushed, this must be removed, and a beveled edged transition should be made to the surrounding sound material.

Once this is done, cut your fiberglass cloth or mat in the shape of the patch you want to make. Damaged areas will need to be built up, so make some smaller patches for the bottoms of cracks, stoved in areas, etc. If the back side of the damaged area can be accessed, grind that area down to glass and make patches for the back side as well. For the boat hull repair, you will need to build up several layers of glass. Use cloth where strength is important, though mat will be easier to shape around compound curves than cloth.

Laying up the glass

This step goes easiest if you have all your glass and matting precut, and you have reviewed the directions on mixing the resin beforehand. For a one square foot patch on a boat hull, you will probably need about 1 pint to one quart of resin, and up to a yard or so of either fiberglass matting or cloth. Mix up as much resin you think you will need to make your patch, and work quickly, but neatly once it is mixed. Brush or roll a layer of resin onto the surface, then place the pieces for the bottom of the cracks in place. As long as the cloth is saturated, don't apply more resin than needed. Keep layering pieces of cloth and resin, keeping the layers of cloth saturated with resin until you are out of cloth. It is very important to also be neat, as air bubbles between layers are a real pain to correct once everything hardens. Clean your tools IMMEDIATELY if you ever want to use them again. If you are doing the repair on the underside of a boat hull, be aware that the new glass will want to sag down under any areas you need to bridge. You can correct this by either turning the boat over, or putting a piece of backing material on the inside of the hull for the new glass to adhere to. Once the resin hardens, it forms a tough but strong composite, and you will have a nice structurally sound repair.

Finishing it Off

The rough repair will be a little ugly, but with a little work you can make it look professional. If you determine that you need to lay up additional layers of glass and resin, you must break the glaze on the new glass, as well as knock down any high spots before applying more glass. In the case where the rough repair is built up adequately, the next step is the same. Using a disk sander with a rubber backing plate, or carefully using an angle grinder, knock down the high spots, rough edges and so on. Feather out the repaired area to the original surface with a medium sanding disk, until you can see and feel a smooth transition between them. To repair any small voids in the surface, and to reduce the effect of the grain of the fiberglass cloth, first wipe any sanding dust off with a solvent soaked cloth, then mix up and spread a thin layer of fiberglass filler, such as Bondo over the affected area, using a pin to pop any air bubbles. Once the filler hardens, sand it smooth as if you were working with spackle. Then prime it and paint it, or use a gel-coat to finish things off.

Making a Mold

Making something out of fiberglass requires you to make a mold. I cannot offer much specific advice, as most of my experience with fiberglass has been doing repairs, but here are a few tips I know:

Positive and Negative Molds

A positive mold is shaped like the object you want to make. You apply the resin and glass to the outside of the mold. They are also fairly easy to make, but you will be left with a rough surface. A negative mold, OTOH, is a cavity where you apply the glass and resin to the inside of the mold. The finished object will be an exact replica, down to the sanding scratches or other imperfections of the shape of the cavity of the mold. For finished pieces, such as the ground effects for a car or other custom pieces, a negative mold is usually used. A positive mold is often used to make a negative mold, so mold making can be a time-consuming two step process, but you can make multiple copies with the negative mold once you perfect it. Molds can be made of wood, cardboard, paper mache, or foam. Avoid making any mold coming into contact with fiberglass resins or solvents out of styrofoam, as polyester resin or acetone will quickly dissolve it. Being careful, it might be possible to make a negative mold out of a fiberglass shell applied to a positive mold.

Once the negative mold is made, a release agent, such as silicone or paraffin wax is applied, so the mold will seperate from the finished piece. A gel coat of pigmented polyester resin is applied as a color coat and first coat to the inside of the mold. Glass and additional resin are then laid up in the same manner as doing a repair. The piece is actually built up from the outside in.

Log in or register to write something here or to contact authors.