Fiberglass for Thermoplastics and Thermosets—These additives increase a part’s strength and ability to withstand frequent blows. But, they’re not as strong or as lightweight as carbon fiber additives. They also block electric currents from passing through. Their short fibers are best for castings and foams to prevent cracking as resin shrinks. The amount you use to start should equal about 10-35% of your composite’s weight.