If it hasnt been waterproofed in 10 plus years and you have access to a Lot of hot water. I really wanted to change the color of my canvas to "match" the new panel I sewed in, seeing as the cheapest and most accessable canvas was a tarp from HF, the Hunter green dosent quite match. I decided to try dyeing the post covers as they were stained anyway and would serve as a proof of concept. They were washed in hot water with detergent and baking soda. There was still some waterproofing present but not much, so I went for it. Regrettably I did not take pictures of the process. I used a bottle of dark green RIT, with salt and dawn. Mixed it up a little heavy on the dye in a 5 gal bucket and 160°+ water. I stirred the covers frequantly for the first 10 min then covered and let it soak stirring on and off for 45 min. They were rinsed well but not washed after this, As I wasnt worried about the dye rubbing off. It came out decent, in my opinion. A little tiedye like, but I like it. I did decided to waterproof the old canvas as is, because I couldnt figure out a time efficient maner to dye the whole thing. Maybe a plastic kiddie pool? And I think you can dye with a spray bottle of salt and dye concetrate. But I did not experiment. I am however happy with the result.