The biggest misunderstood thing with FRP fiber reinforced plastic, is it is the fiber that is the strength NOT THE RESIN. So, using a factory produced fiberglass shapes (Rod, strip, tube) is your best bet. These are pultruded, (the opposite of extruded). The fiber, (glass, carbon, Kevlar) are drawn out of a die (steel tool with a hole) and the resin is infused into the fiber before it leaves the die, The result is a solid mass of fibers, with just enough resin to stick them together. This is one way to utilize the most strength the fiber has to offer.
98% of the hand layups I have seen in my 30 years of movie work, would get you fired from a high end composite shop. The miss information circulated in the EFX business and the amateur prop community is disheartening.
If you make your own FRP, polyester resin or Epoxy resin, make sure you practice on a test piece (or three) and get to the point where you can use the least amount of resin, but still "wet" out your layers of fiber and leave no air bubbles. This takes practice, but even more, you have to know this is the best way to get the strongest part. If you listen to most "Hobbyists" you will be adding too much resin, and/or cut your fiber pieces too small. When wetting out the fiber, make sure to only mix enough resin to work small areas. Exothermic resins (Polyester and Epoxy) will set up faster in larger masses. So the resin in the cup will set up (harden) long before the resin in you layup. Mixing too much resin, just causes you to try and rush the process and wastes resin. Start in the middle of your part and work to the outside. This makes the fiber move around to match your mold shape, as the outer, cut, loose, fibers can freely move. Starting from the edge, sticks these ends down and the glass won't want too slide around as easily or it will want to keep "springing" up. Compound curves are pretty easy with cloth or mat. But tight 90 degree angles make things more difficult, as the glass is stiff and doesn't want to bend that sharply. (This is why glass and carbon are so strong).
This is why:
Vacuum bagging is a way to compress the fibers using atmospheric pressure the squeeze the fibers as close together as possible and a barrier layer and an absorption layer suck up the excess resin. Using carbon fiber with out vacuum bagging is like buying a $200,000 Ferrari and only driving it on a dirt road. You have spent a great deal of money for a performance advantage you will never see. High strength carbon layups are done in an autoclave., a heated pressurized oven. The compacts the fibers even more and the heat sets the resin in a controlled manner. This is where you get "stronger than steel" properties. You can't get these in a hand layup.
Fiberglass shapes are available at McMaster Carr and if you have a sailboat manufacturer or dealer, sail stays or battens, are a great source for premade FRP shapes.
