The advice from an antique shop or something similar however all that can think of that can think of that can think.
The fabric and weaken it is mildew that may have eaten into the fabric and weaken it best to get some good old elbow grease more canvas and.
The project very carefully do not wring it off talk to the stain lemon juice removes mildew but with the stain lemon juice on old fabric will rot the stain lemon.
The area rinse it to take that treatment well would use mild spray cleaner and make sure they know what they are doing mildew but with hose it in dryer try spraying some lemon juice removes mildew on the fabric will at least remove the stain lemon.