He had two children with his first wife and a further fourteen with his second wife, Mary Anderson.
In 1740 William started a weaving business with his brother Robert but as the duties of the church became ever more demanding, Robert left the family business to concentrate on the church. William then took on another partner, Hector Turnbull. The two families were united even more with five of William's children marrying five of Hector's twenty children !
William was said to be "renowned in the annals of the burgh [i.e. Perth] for his 'indefatigable industry' in diverting and excavating a canal through granite for manufacturing purposes." It seems, in order to avoid his bleaching fields becoming flooded, he moved the course of two rivers and created a two mile long canal, some of it built through granite, to service his mills.
In 1752 William leveled 12 acres of bleachfields in Luncarty. A bleachfield was usually a field which was covered in cloth and fabrics so they could be bleached by the sun and water. This was an integral part of textile manufacturing during the Industrial Revolution.
The Stanley Mills continued to produce textiles for over 200 years, until it finally closed its doors in 1989. Now it stands as a unique example of Scottish industrial history. There is not just one mill at Stanley, but rather, a complex of mill buildings beside the Tay. Among them is The Bell Mill, one of the oldest industrial factories in the world, still largely unchanged from the day it was built. The Bell Mill now hosts interactive displays which tell the rich history of Stanley Mills. There are exhibits which let you test your on dexterity and let you experience what it would have been like for the young children who helped keep the mill running. Beside the mill buildings is the village of Stanley itself, built during the 1780s to house the workers at the mills. (Source : Britain Express)
By 1790 when William died, the Luncarty bleachfields covered 80 acres and processed 500,000 yards of cloth annually. He was buried in the Greyfriars graveyard in Perth with the inscription "William Sandeman - manufacturer Perth and bleacher at Luncarty".