I lightly mist the ganached cake with a spray bottle (on mist setting, not spray or jet setting!), containing cooled, boiled water. Others just brush the water on too. Ensure your tiny amount of water covers all areas of your ganache, otherwise you it will not stick properly and you will spend a good few minutes popping air bubbles with a pin (ask me how I know ).
Oh dear, those pesky bubbles! I've had problems in the past with bubbles when I'd put fondant on buttercream, finally I learned to not put it on a cake that's been in the fridge for 6 hours.... Definitely want to avoid the bubbles! Off to get a spray bottle! Thank you!
I typically rub mine with a light coating of shortening...
I rub my shortening on my fondant. This seems to help my fondant from drying out before I can get it smoothed, reducing the elephant skin phenomenon while providing a 'glue" for the fondant and ganache.
So I started using ganache under fondant, and got a major bubble issue. First I tried vanilla vodka lightly spritzed on to attach the fondant... major bubbles, twice. Second, I tried water lightly spritzed on... major bubbles, 3 times. Last I tried a thin coat of shortening I rubbed between my hands then rubbed onto the underside of the fondant before attaching... only 1 bubble! My vote is definitely for shortening.
When you mist the ganache, no matter how careful you are, it will bead up to a certain extent. This allows more air to be trapped under the fondant. When you rub shortening on the fondant, you're getting a more even coating and less air, so fewer bubbles to deal with.