It's a very good sealer to use for underbody and wheel arch protection, but to do an entire sapphire it will cost a small fortune! You would need at least one bag per arch and a fair few to cover the floor. I would argue it's one of the best products out there so it depends how far you want to go with it.

Don't use it to cover any mounting areas but otherwise I'd use it everywhere else. It's also overpaintable I seem to remember.