To make a durable composite laminate you need to have both a reinforcement (fiberglass cloth, fiberglass mat, carbon fiber, aramid, etc.) and a resin. Resin holds the reinforcement together and helps it conform to the wanted shape. Resin fills the fiberglass weave. Just like there are several reinforcement options there are also a few resin options.
The most commonly used resin is polyester. It is more economical than vinyl ester and epoxy. It is the easiest of the three to use. It has a quick cure and adds dimensional stability. It is often used in building/repairing boats, car bodies, patio decks, sporting goods, surfboards, kayaks, decorative surfaces, outdoor ponds, bath tubs, aircraft, plus more.
There are several grades of polyester resin. The most popular is the Ortho General Purpose Laminating Resin (GP Resin). It is used for general fiberglass applications. It cures with a surface tack which helps the multiple layers adhere well to each other. It also holds the reinforcement in place. Another benefit to the surface tack is that it is not necessary to sand between layers. On your final layer, wax can be added (surface agent or surface seal) to the resin. Or a Finishing resin can be used. There is wax in a Finishing resin that rises to the top when cured and seals off the air thus providing a hard finished surface. There will be no tack.
ISO Resin is a superior grade polyester laminating resin. It has higher heat distortion, is more impermeable to moisture and has better corrosion resistance. It also has a better tensile strength. It is very commonly used in mold making because it is a stiffer resin and less likely to distort. It is also used on pipes or parts that require the higher corrosion and temperature resistance.
Surfboard Resin is another popular polyester. It is a water clear, UV and impact resistant resin. It provides some flex to help resist impact damage. It also has UV inhibitors to protect the water clear appearance.
MEKP is the catalyst needed to cure polyester resin. This should be done at room temperature. Add more or less catalyst depending on how long of a pot life and working time is desired. Pot life is the amount of time it takes before the resin hardens in a mixing cup. The ideal temperature to work in is 70 degrees. It is unlikely the product will cure in temps under 60 degrees Farenheit. The product could cure too fast if the temperature is above 80 degrees Farenheit. Typically catalyst is used between 1.25% to 1.75% (1 2/3 ounce to 2 1/3 ounces per gallon). The less MEKP added, the longer the pot life/working time will be. The more MEKP added, the shorter the pot life and working time. Be careful though. Too much catalyst and the finished product can be prone to fractures or the resin in the cup will form a rubbery material before it can be used. If not enough catalyst is added, your resin will never cure. Cure time can be affected also by how thick the product is and how much resin is mixed per batch. It is best to work in smaller quantities.
When working with resin, be sure to wear rubber gloves and a mask. Work in an open area as polyester resin can have strong fumes until it is cured.
To make more of a putty or to thicken the resin, there are many different fillers available to purchase. They include: talc, wood flour, milled fibers, glass bubbles and fumed silica. Fumed silica (cabosil) is the strongest, but hardest to sand. Glass bubbles are not very strong, but easy to sand. Depending on the desired strength of the putty, use one or the other.
Do not use polyester or vinyl ester resin on Styrofoam. The styrene in the polyester or vinyl ester resin will melt it. Epoxy resin should be used.
Resin has a short shelf life. It is one year depending on the climate it is stored in. Try to store the resin in a cool climate, or refrigerate (do not freeze) it to extend the life of the resin.