I had the same issue on my 602 after a couple years of use, except the dust on mine was and still is limited to the side corners at the rear of the stove. I posted a question about it here and replies were mostly that it didnt look like more than normal rate of deterioration of gasket cement. From the picture however, the ash on yours appears more extensive than mine along the bottom edge. Perhaps try using a lit incense stick to test along all seams for leaks while the stove is operating, the wisp of incense smoke more precisely reveals leaks than a match flame. If leaks are present but only at a spot or two, you might be able to buy some time before having to rebuild by spot sealing the leaks from the inside with cement.