To be able to effectively work with the fiberglass, you need to follow the strict process. First, use a sandpaper to smooth the edges of the foam until flat. You also need to make sure that no dust is attached to the foam because these unwanted dust will affect the whole process.
Next comes the second stage: cut the fiberglass mat. A razor knife may come in handy in this case and remember that cut the mat to the shape and size you desire.
Wet the foam with resin and then apply one layer of fiberglass mat to the foam one by one, one hardens then reapply the other. Just like that and you’re done.