Long story short, we added some porches to our new home that are over living area. In order to make sure that they don't leak, I had some friends who build custom offshore fishing boats come up and fiberglass the decks. Now we're watertight but we're struggling to find a way to lay the tile over the fiberglass.

Our tile guy has never worked with fiberglass and is unsure about what to use that will adhere to the fiberglass. He says that thinset won't stick to the fiberglass. He suggested a product called Latapoxy 300 but it's ridiculously expensive. Just wondering whether there are any tile people on here that can suggest a better, and less expensive, material or method to tile these decks?