My new step project at my house is underway, my first. Looking ahead, I'm wondering what the best way to fill the joints between the pieces of granite on the top platform will be. There will be a full width granite tread at the start of the platform and the rest will be granite pieces in 1x1 and 1x2 squares. I thought of using poly sand like I would on a walkway but it would run out the sides where there is no border. Any thoughts?