I have been able to clean out many stubborn stains with starbrite's nonskid deck cleaner with PTEF. squirt it on, lightly scrub (more spreading than scrubbing) with a stiff bristol brush, wait about 30 seconds to a minute (Do not let it dry) then scrub. The cleaner will get really sudsy as you scrub. Then just rinse it off. (Use only for the deck, I doubt using deck cleaner on canvas would be a good thing)
best of luck!