Like Fishstik says, you're not drying the fish properly. After rinsing the brine and paper toweling the fish, I put mine on the racks and then put a fan on them for at least two hours, longer for real fatty fish like springers. You want the flesh to be dry to the touch, with a shiny surface known as a "pellicle." You can also brush away the tarry stuff off the top of your smoker, but you shouldn't have to if you dry the fish properly.