FRP, or Fiber Reinforced Plastic, is an ingredient to make the Boat in both stage 1 and 2.
FRP is used in the following recipes:
Where to Find
Stage 1: Once you have a Raft go to the island directly left of the main Island. Similar to the Iron Ingot, FRP is obtained by using a shovel on a hole under a Potato bush on this island. The hole looks identical to the one the Iron Ingot was in so it should not be a problem to find it once you are in this area. It does not respawn.
Stage 2: The FRP is in the locked gate room in the cave on mine island. To get the code you need to use the drill on a wooden door with the ‘stop’ sign in the cave on mine island.