The Coarse grind is the shaping step of your tumbling process. Sharp corners, rough edges and pits are removed in the coarse grit cycle. Stones that have been crushed have many sharp points and sides to them, and may need several coarse grit stages to get them adequately rounded. Beach stones that are already round may have pits from the forceful natural tumbling process and should still go through a coarse grit cycle to smooth the shape.
Enough stones should be used to fill your barrel to about ¾ full. Less and the action can be too fast causing chips. Too full and the action is too slow, and will not accomplish much. The water level should just cover the stones. Also, for maximum tumbling results a filler is a good idea. A filler can be plastic pellets or small stones. It is best if you use stones that are of the same type and hardness of the larger stones you are tumbling as a harder filler can scratch softer stones. These filler rocks should be small, between ¼ and ½ inches. They help carry the grit, and rub it against the larger stones. We recommend 1/3rd to ½ of your batch be filler. Your filler can be used over and over. Just keep adding more to keep your barrel ¾ full. We use 1 tbsp.* per pound of rock. (3lbs. tumbler = 3 tbsps of grit) More than that is not used up in your week of tumbling. When you are changing grits and cleaning your barrel, if you find a lot of grit you have added too much.
When looking at your stones after the first coarse grit stage, if there are some that seem to be a lot smaller, they are probably softer than the rest of the batch and should be removed and tumbled in a separate batch of like stones. Look at the shapes to see if you want to put them in another coarse grit cycle or move them on to the medium grit cycle.