I use Plexus myself and have had great luck with it. Regarding the grit issue, I suggest that you use the Plexus after some other form of rinse-off cleaning... for the exterior, I just do the Plexus application after a car wash. For the interior, I use a spray bottle filled with water and a cotton towel "turned frequently". Then I follow with the Plexus via the "soak the applicator cloth and wipe dry with soft cotton towel" method. Applying Plexus without removing the grit prior to the "rub-in" could be counter-productive...
I've noticed several folks around here using small pillows or towels inserted as they lower the top to help protect against scratches from the window rubbing as the top is stowed... seems like another good possible solution to the grit issue between washings.