General information
Fiberglass, fiberglas, 'glass are all terms used to describe a composite building material that contains a fibrous material that is saturated with an eventually hardened resin of some sort. The fiber component (usually finely stranded common glass, others discussed below) give the composite amazing tensile strength but, being a cloth, has no ability to hold a form. Thus the need for the resin which when saturated into the cloth is hardened with a chemical reaction giving the system strength against bending and shear forces. Not only does the resin give the composite form, it also is used to provide a surface for the desired finish of the product. Each component contributes a unique quality to the final composite and the whole is better than the parts.
The following sections discuss some items that should be helpful to getting started in using fiberglass on your projects. Once you start using it and get comfortable with it, you will wonder how you ever lived without it. It is FUN!!!!
 How much mat (or cloth) to use
You may wonder how many layers of mat to use; what kind of mat or cloth to use, etc. There isn't a single answer for all parts. The shroud shown in Make a fiberglass fan shroud is mainly for form and has little strength requirements, so it can be laid up with just the chopped strand mat. However, if it were a body part or some other part that would see some stress, the design should include one or more layers of woven cloth which is many times stronger than the mat.
 Types of fiberglass
 Chopped strand mat
Chopped strand mat is rated in ounces/sq ft. It commonly comes in 3/4, 1.5 and 2 oz/sq-ft thicknesses. The 2 oz is probably most popular because it builds thickness quickly. The lighter thicknesses are obviously for smaller, more delicate applications. For most car parts, use the 2 oz. This material should always be used as a first layer in a finely finished mold such as a boat hull where there is a gel coat layer (final finish colored resin layer) then the 'glass. If you lay a layer of woven cloth directly under the gel coat it will 'print' through the gel coat and you will always see the pattern of the cloth no matter how much you sand and polish. The random pattern of the chopped mat strands isolates the cloth pattern from the final surface. Again, in a "male" mold, this isn't a consideration.
 Surface veil
There is a special type of chopped strand mat called veil. This stuff is very thin and lightweight, and is used as the first layer under gel coat, even before the 2 oz mat, and does even a better job of providing a finer final finish in the gel coat. It gives no strength or bulk, just a finer finish. It is recommended where gel coats and polished female molds are used.
 Woven materials
The woven materials come in a confusing array of weights and weave patterns, all with a special purpose.
 Woven roving
"Woven roving" looks like your great Aunt's front door mat. It is woven with very coarse strands in a 90-degree pattern, and its purpose is to build bulk fast while providing superior strength compared to chopped strand mat. It is quite thick, and due to its weave and big strands doesn't lay around corners, and doesn't bond well to other layers. It also loves to print through its weave pattern to ruin gel coats. It should be used in boat hulls, not car parts. If you insist on using it for building thick parts, always lay it with alternate layers of 2 oz mat to improve bonding.
 Woven cloth
"Woven cloth" is the star of strong fibre-reinforced-plastic (FRP) construction. It strangely is sized in ounces per square yard instead of square foot and comes in weights from 1/4 oz per square yard to 10 oz per square yard. It comes in "plain weave", "satin weave", "8-hs weave", and possibly other patterns (confirm and expand). Get the plain, it's plenty strong and cheaper. You can usually use 1.5 oz, as this is what you want for 99.99% of your high strength layups.
 Other fiberglass cloths
"DBM" is dual bias mat roving, which is two roving mats stitched together with the weaves @ 45 degrees for more strength. For the boat guys, not for hotrodders.
Also called "carbon fiber". Really exotic, looks purty and necessary for the 350mph crowd, not so much for shade tree guys. To be purty, must be laid up in vinyl ester or epoxy resin in shiny female molds. Is fairly brittle so commonly has a layer of aramid cloth or fiberglass for toughness. This stuff is sized in thousands of filaments, i.e. 1K, 5K, 12K, etc.
 S-glass or E-glass
These are high quality woven glass that is measurably stronger than regular woven fiberglass. More expensive and not necessary in most of hotrodding.
E-glass is actually the commonly found fiberglass that you would pick up at a boat supply or hardware store. S-glass is the higher quality fiberglass mentioned above. Typically, E-glass is not labeled as such, it is the S-glass that is always prominently labeled.
Also called Kevlar, this stuff is amazingly strong and tough. It is so tough, it is almost impossible to cut! It is used as strength enhancer in very light weight applications that need extraordinary strength. Again, it is overkill for just about anything in hotrodding.
Like it sounds, this is a specialty cloth that has most of its strands in one direction. It is sewn together and for special applications. Can be made of any of the above threads. Not necessary in hotrodding.
 Types of resins
 Polyester resin
Polyester resin is used for 99.99% of hotrodding tasks.
 Hardening time
You can adjust the hardening time of polyester resin somewhat by altering the amount of MEKP hardener you add. The more hardener, the faster it sets. You can add too little hardener and it will never set. However, there is a wide range of hardener concentrations that work fine. If you have a piece that isn't setting due to cold weather or short hardener you can set the piece in direct sunlight. UV radiation will set off this resin seems like no matter how much hardener is used. Be careful with this though, the resin setting chemistry gives off a lot of heat and too thick a part, too much hardener and/or solar acceleration can get the thing smoking and even start a fire!
 Types of polyester resin
There are several different types of polyester resin used for fiberglassing.
Made with orthophthalic acid, ortho is what you get if you don't specify anything else. Fine for most everything in hotrodding.
Made with isophthalic acid, it bonds a little better than ortho but costs more. Again ortho is fine for most hotrodding work. An added benefit to using "iso" resin is that it is more heat resistant than ortho resin, it will withstand 210°F as opposed to 170°F for most ortho resins. Iso resin is also called "tooling" resin, because it is designed for moldmaking, and can withstand many heat/cool cycles and remain dimensionally stable.
 Waxed resin
Be sure you check whether you have "laminating" or "finish coat" resin. The former is just plain resin, and is used for laying up several layers. Every layer will stick to the previous one very well. The latter contains a wax that floats to the surface of the finished part and, if used in laminating layers, will prevent the layers from bonding, causing part failure. These two resin mixtures are necessary. Polyester resin will harden all the way through except for the very outer surface which is exposed to oxygen. This exposure prevents the resin from hardening and results in an irritating sticky feel to the surface. Using the waxed resin in the final layer allows the wax to float to the surface, insulate the surface from oxygen, allowing it to fully harden. One option is to only use laminating resin, and sand off the outer surface, or prime it, which seals and hardens it. No biggie, don't worry about it, just be careful to not laminate with finishing resin! Another common name for this resin is "sanding resin".
 Gel coat
This is resin filled with a high solids, usually colored pigment. It can either serve as a primered surface to be sanded and painted or as in the case of boats, can be the final colored finish. It is sprayed in a polished female mold without reinforcement, then 'glas is laminated on it. It is much softer than plain resin and works like a sanding primer. Good stuff.
Stronger than polyester but more persnickety and expensive. Also check with the supplier for compatibility because some cloths won't work with some resins. This can be used for looks if making a carbon fiber part. Epoxy resin is also tough on your skin and respiratory tract. It's also much harder to work with; vacuum-bagging the parts when laminating is commonly used with epoxy, because it is so much harder to remove all air pockets with rollers or squeegees.
 Vinyl ester
This is a stronger version of polyester resin, and somewhere between polyester and epoxy in strength. It has the ease of workability of polyester and compliments the strength of fiberglass much better than polyester. Not usually needed in hotrodding. Vinyl ester is much more heat resistant than either of the polyester resins, and also chemical resistant. It also shrinks a lot less, so print-through is less of a problem. If you want to easily make some very strong, rigid, and lightweight parts, use vinyl ester resin, along with either fiberglass mat or cloth, and add a layer of 6 oz carbon fiber in the middle of the laminate.
There are also many cores available for use in FRP laminates, use of a core can increase the rigidity of a part without increasing the weight or tendency of thick parts to get mottled after time and exposure to heat. Use a core anywhere you have a large flat or gently curved area, roofs, hood tops, floorpans, bellypans. The underside of fenders above the tires is a good place to use a core, as it will provide a cushion against stars from rocks being thrown up by the tires.
Some of the cores you can use are:
 Balsa wood
This is used in large flat areas and needs to be bedded in...not really a good choice but it is one of the oldest cores. Boat builders used it extensively for reinforcing large flat areas such as decks, hull bottoms and cabin roofs. It's even used in the floorpans of Corvettes, for stiffening the floor (, , ).
This is a material that looks similar to the absorbent paper shop rags that come in a roll. It's made with fiberglass and microballoons (more on 3-M microbaloons, aka microspheres HERE, along with a binder. It's much more flexible than balsa, and it comes in several thicknesses: 3mm, 5mm, and 8mm. This you lay up into your part, just like another layer of mat or cloth, then put another layer of mat over the top.
 Urethane foam
This comes in varying thicknesses, and is used much like balsa wood. Not only is it good for filler material in a fiberglass layup, it is great for carving molds for parts. Folks have even carved entire car bodies from urethane foam blocks then used that for making a female fiberglass mold which in turn is used to make production car bodies.
 Fiberglassing procedure
 How much mat to use
As far as how much to use, it depends on the part you are making. For a fan shroud you can lay up two layers of mat over most of it and reinforce all the edges with a third layer. All 2 oz mat. This would give a very light weight body, a little less than 1/8" thick and stronger edges a little more than 1/8" thick.
A body part would need a couple layers of 2oz mat, a layer of 1.5oz plain weave cloth and possibly a third layer of mat, depending on size. Again, reinforce edges as required with a strip of mat.
For one example, see this journal entry from Willys36. It's a shell for the headliner of a '53 Chevy pickup, and used only 2 layers of mat (about 3/32" thick) for light weight and flexibility. It is more than strong enough. This is also an example of a part made from a sort of female mold (the roof of the truck), but without the gel coat since it is covered with upholstery and doesn't need a paintable finish.
 Reinforcing edges
Reinforce edges as required with a strip of mat. You can also reinforce edges with woven tape, this puts many continuous unbroken filaments parallel with the edge of a part, and will make it much more resistant to cracking from the edge in.
 Related articles
 Fiberglass health and safety
- Synthetic Mineral Fibers -- Health Hazards, US Department of Labor, Occupational Safety and Health Administration (OSHA), retrieved June 26, 2007.
- Facts About Fiberglass, American Lung Association, retrieved June 26, 2007.
- Fiberglass: Environment and Occupational Disease Epidemiology, New York City Department of Health and Mental Hygiene, retrieved June 26, 2007.