Hi Jeremy
I doubt whether you will be able to bend FRP to 90 degrees. I have worked in the GRP trade for about 20 years, bends of that angle would be molded into the product during manufacture. A GRP sign company I worked for used to bend bits of GRP by heating them up in water and pressing into a mold which was just a slight arc. It may be made from a special resin (plastic) as there are many types with different properties so not saying its impossible, but I don’t think you will have much joy trying.
Steve