Demon WP is a wettable powder. It dilutes with water and when it is sprayed and dries, it leaves a powdery residue. This residue can appear like fine dust on dark surfaces, especially tile and wood. I recommend to only use 1 sachet per gallon of water when spraying indoors. If you have dark surfaces and the Demon shows up on it, simply wipe off with a paper towel. It comes off easily, and does not leave a permanent stain in most cases.