You only need a very thin layer of epoxy, maybe max 1/16" thick in each of two layers, one on the transducer itself, one on the sanded hull. Any more just increases the chances of air bubbles and each one of those either block signals or cause noise. Get the dealer to let you borrow a new transducer to test the setup on the water. First hold it outside the boat, belly aimed down. If the display is clean then put some water in the bilge and put the transducer in the water, look for a clean display.
Jim
Jim