Smooth insides of duct
Inside each side's duct, there will be a small ridge where the factory join was made. I smoothed this off by taking a 5" length of 3/8" sq balsa and wrapping some 100 grit paper around the end. The result is kind of like a pencil-top eraser that is mildly abrasive. This allowed me to reach well inside the duct from both sides, removing the highpoints. Then I ran a bead of plastic CA down the inner join line to fill any small depressions and sprayed a thin coat of primer to show up any remaining imperfections.
The photo below shows the inside of the duct after this first step. The process will be repeated once more to get rid of the remaining low-points. (Note - this is not strictly necessary - it's just cosmetic)