Fiberglass Polyester resin capabilities

• Fiberglass polyester resin is relatively weaker than epoxy. Tested to about 500 pounds per square inch, this is a fourth of epoxy's 2,000-pound test strength. For most fiberglass construction projects, however, (boats, sporting equipment, building materials), 500 psi is more than adequate. The only instance of fiberglass construction the home hobbyist should avoid using polyester resin to bond fiberglass with would be constructing a manned airplane from a kit.
  
  

Mixing

• When using fiberglass polyester resin, you must mix each batch before application and use within about 20 minutes after mixing. It sets up hard and fast in less than a half hour. Any unused portion of mixed resin must be discarded.
  
  Use paper cups or other disposable open containers for each batch. Two ingredients are mixed together to make a fiberglass polyester resin batch: resin and hardener. It's not possible to list all the possible ratios for mixing these two ingredients because it varies with heat and humidity. Generally, temperatures of 72 degrees Fahrenheit and a 50-percent relative humidity is a 15:1 mix. As temperatures fall the resin amount goes up. Add more hardener for higher temperatures. Resin manufacturers will include a table on the product label describing the ratios for particular conditions. It is best, however, to apply resin in moderate temperature and humidity conditions. This avoids fiberglass polyester resin from taking too long to set up (or not at all) or too soon. It is possible to mix a batch of fiberglass resin and have it harden into a solid lump before you get a chance to apply it.
  
  

Application

• Treat the fiberglass resin as you would a thick paint or varnish. You can roll it on, apply with a brush or use a spray gun for large areas. When applying fiberglass resin avoid areas with any dust. Contaminants settling on the wet fiberglass resin will mar the finish. Unlike paint, it's almost impossible to clean out a speck once it's in the sticky hardening resin. Also, just remember to have plenty of acetone available to thin the resin and clean your brushes.
  
  

Color

• Fiberglass resin coloring comes in small bottles or tubes and is used similarly to food coloring: a few drops to completely color the fiberglass resin to opacity. There are also color agents available that will color with a translucent hue and offer some transparency to the final fiberglass finish.