I just finished all the repairs this morning for people to come and get before I close shop to go fishing. The one boat cover I was avoiding got pulled out of the corner and it made the shop stink. I think it was under a car during an oil change. It had that burned oil stench and was quite dirty from gravel road dust. While it was on the sewing table it made an awful mess that had to be cleaned off. On top of that I had to spray WD40 through the parts underneath the sewing machine to get the dirt out.
Most of the things brought in for repairs are not as bad as the one I just decribed. This customer is going to pay extra for the time to clean things up after working on his cover and that was expressed to him when he dropped it off. He also now knows that if he simply washes it down with water and a brush it will last alot longer.
So if you want your cover to last, clean it off once in awhile. Don't use anything harsh on it. Usually plain water and a sponge or soft bristle brush works quite well. Many common cleaners will damage the UV or water resitant coating on fabrics and cause premature fading or even cracking on coated fabics. If you need to use something for stubborn dirt, mold, mildew etc. I can recommend some products that will clean without damaging the material.
If you plan to bring something in for repairs and its dirty, please clean it beforehand. Your canvasman will apreciate it.