Yeah, I have covers on the vettes under the carport as well. Never had any sand underneath of them though. Scorpions, yes, but no sand....
I always break out the leaf blower and blow off the cover before taking it off of the car. This helps to keep grit from getting on the inside when I roll it up off of the car.
I may get a slab poured for the carport, though. It's sand underneath so if I drop a microfiber towel, it's basically history.
Next time I get this sort of an idea, I'm going to go to a contractor instead of talking to the metal building manufacturers. I just couldn't get a real good feel for the final bottom line talking to them. All they want to do is to sell me the building and be done with it.
I guess I could go ahead and have that area cleared and put the slab down. Kind of do it piecemeal, having the garage then constructed on the slab when I am ready for it.