sanding dust is always a mixture of wood and whatever aggregate is used on the sandpaper. that grey tone might be the curl filled with this mixture as the sandpaper granules break down. always use the best quality paper find and make sure it is new and sharp and replaced often. over sanding with dull paper will set the dust in the curl. remember sandpaper is nothing more than a whole bunch of cutting edges on a flexible base,.....one of the first rules of woodworking is to keep your cutting edges sharp. secondly, if you've put a sealer of any kind on the stock. sanding without frequent dust removal, will push the dust into the end grain of the curl,....this dust is usually somewhat gray colored. keeping good paper around to use can get expensive and the temptation to save a few bucks is strong, but paying good money for good sandpaper is always the way to go. also, using wet or dry without some lubrication and frequent washing clean of the area being sanded, will produce this greyish dust, because the lubricant washes the dust out from under the sandpaper leaving it clean and sharp to make good cuts. all the above is the primary reason scraping is the recommended way to finish curly woods.
