You stated that the dust was grey in color. When I first started burning my Jotul 507 I had grey dust all over the living room where the stove is located. I thought it was fly ash, however, after paying attention to the stove I found out that it wasn't fly ash as my fly ash was a dark brown color; it was coal dust. I was not being careful and mindful of the dust I was creating while loading the stove or bringing coal into the house. I no longer bring the bags of coal into the house and dump them into my coal bucket; I fill the bucket outside and I am careful when shoveling the coal from the bucket into the stove. Once I started this practice I no longer had grey dust. As far as fly ash in the house if you have a good fire going the draft will pull the fly ash up into the stove and not out the ash door. I pull my ash pan back even with the door opening and by doing this it makes the door opening smaller yet I can still see the grates. By doing this I do not get any ash falling out the door or fly ash. All fly ash is pulled up into the stove. As far as your wood working, your probably getting wood dust into the house. As an owner operator of a high end wood furniture shop I deal with the fine wood dust your creating in your garage. The only wood I have ever worked with that can create a grey like dust is purple heart. There are a couple of other exotic hard woods that might possibly create the grey dust, however, I still believe that your grey dust is a very fine layer of coal dust. I'm not saying I know anything or that this is your problem or will solve your problem; this is just my experience with my coal burning setup.