If you do not want the silicone to show (and it will!), use the minimum that you can to do the job and then cover the silicone with sand while it is still wet. Try a few small sample pieces to get a feel for how much sand you need, and then view them underwater to see the results. I know that sounds stupid, but something about being underwater makes the silicone show up much more than when it is dry. I learned this the hard way.