Re: Bennington seats
I just mix up a batch of auto detergent like you use when washing your car. And use some elbow grease and sponge.
If any stains are still there, I'll usually just go to Awesome--the incredible cleaner sold @ Dollar General for $1 per spray bottle. It's the only thing I've ever found that'll remove black streaks from my travel trailer.
I have some 303 cleaner, but it's no better than other cleaners. The 303 protectant is the stuff to use when you get the seats clean.
My problem is with spider droppings from the ceiling of my boathouse. I put my OEM cover on the boat, and those devils still get under the cover. My next step is making canvas seat covers to place over my seats when not in use.