I second the comments about the bedding compound needing to be flexible so you keep the water out.
A tip for applying it: Apply the bedding compound to the areas as needed. In addition to the exterior edges of the fitting, I always put a circle of bedding compound around each of the fastner holes as well. Putting a little compound around the head of the fastner and on the inside surface of the washer won't hurt either.
With all the compound applied put the fitting in place and run the nuts onto the fastners getting them firmly in place but not as tight as you want then finally. Clean up the excess compound with an appropriate solvent and let the compound set up. By not tightening the fastners all the way down you won't squeeze all the compound out of the spaces between the fitting and the deck. Once the compound has set up, you can come back and torque the nuts down hard. With the compound set up, this second tightening compresses the compound and results in a better seal.