According to the folks at Reliable Canvas tents in Bozeman, MT ; use one part bleach 3 parts water.
The mold is what destroys the cotton fibers and depending on how bad it is will depend on whether you can clean and restore...if its been eating away at the tent for any length of time- its probably toast.
They have heard of some folks using other things like Oxyclean...but it doesn't always kill the mold....the mold sometimes comes back when it gets wet again.
Worth noting; Folks using straight untreated 12oz canvas will have a tougher time with mold as without the treatment they get moldy easier, Reliable always recommends going with the Treated canvas over untreated.
I just replaced my 18 yr old canvas wall tent on their 15% off sale. Mine was still waterproof, but wearing thin in places and ripped from pulling on it- it was time, its been used a lot.
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