How was the wood exterior of Mount Vernon made to resemble stone?

Answer 1

According to the recent book "At Home" by Bill Bryson, George Washington faced his house with planks of wood which were chamfered at the edges to look like blocks of cut stone and painted to disguise knots and grain. While the paint was still drying, sand was gently blown on to them to give a gritty, stone like texture.

Answer 2

The long pine boards were cut into a brick/stone shape then sanded smooth and painted. While the paint was still wet, fine sand was thrown onto the boards to create the stone effect.