The charge to paint a building is exempt from sales tax. If your charge includes the material and the labor, the entire charge is exempt from tax. But, if you have one invoice for the labor and a separate invoice for the paint – the labor is exempt from sales tax but you will need to collect and remit sales tax for the paint.
The painting of a building is considered improvement to real property which is exempt from sales tax. You can read more about real property improvements here.