I just purchased a used 14'x16' wall tent from an exploring company. The tent is only 2 years old and the canvas is in great condition but it is dirty. The back wall is dirty from the diesel stove (soot and a few splash of diesel) and the front door is dirty from what I assume is people brushing against is with muddy jackets as they were coming in. What is the best way to clean it without removing the mildew/waterproof/fireproof coating?