alright, i work in a boat yard and we build and repair boats with composites.... aka carbon fiber, fiberglass, kevlar, ect......if you vahe composite decking it is most likloy fiberglass, becuase its cheap to buy and easy to work with.....the problem with grinding on fiberglass is that metal edges will eventually start to dig in, and unline metal, fiberglass will crack, compress, and turn into power as you continue to grind, it will be very difficult to grind it for a long period of time because chips/cracks will cause you to catch and edge and eat shit.....it may be fun the first few times, but its not durable and is certainly not long lasting