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I've had mine outdoors for about three years, and all I do is wipe it clean with a damp towel, dry it off, and apply Armor All. I do this about three times a year. I don't have a zipper on my cover, sounds like it would make things easier.
I ordered one after seeing what the weather has done to the insides of my grill.. It seems well made and while I have nothing to compare to, I like it a lot.
I have a cover for a Weber kettle that I use on my GOSM- covers the top down past the handles- The guy I bought my GOSM from tossed in the cover as part of the sale. My GOSM has a home on my covered patio so I don't use the cover much anymore.
I have one for my big block that I have had for a couple of years. It does not have a zipper but has velco strips to go around a couple of the legs. It is thin but it serves it's purpose.