Re: Thumbs up for 303 Fabric Guard!
No reason not to apply it in place. I have used it for years on an old Mercedes SL convertible top - dark brown German cloth type. I just wash the top in usual manner, then spray one 16oz bottle on it once a year. I do cover all of the surrounding surfaces just in case, but no big deal. The current top is about 15 years old and looks new, although it doesn't get the exposure it would on a boat. I just learned here that it comes in gallons - good news, as I am doing my cockpit cushions, sunbrella fabrics, etc. over the winter.
1985 Marshall 22 catboat