We think of anything covered with fibreglass as being almost invincible, yet whether it is a fibreglass boat or a deck, this mixture of glass fibres and epoxy resins has one unrelenting enemy: UV rays from the sun.
In the photo you see what a fibreglass skin looks like when put over wood, in a manner quite similar to what is done on boats. It becomes transparent and makes the wood look pretty nice. Yet if you do not paint it with something that blocks the UV rays of the sun, it will not stay stuck for very long. That is why fibreglass windows, boats and decks are usually made with coloured resins, to block the UV.
If you do want that transparent look, you must paint it, and maintain the coating, with a clear marine varnish that has UV blocking filters in it. Then you can leave it transparent.