Identify every surface
Concrete, brick, siding, wood, roofs and painted surfaces do not all tolerate the same pressure or chemistry. Walk the property and ask how the contractor will treat each material.
Ask about preparation
Discuss water access, moving furniture, protecting outlets, closing windows, covering sensitive plants and controlling pedestrian traffic. Confirm who is responsible for clearing the work area.
Understand runoff and chemicals
Ask what products will be used, how they are applied and how runoff will be managed for the property. Requirements vary by site and location, so the contractor should be prepared to follow applicable rules and your facility procedures.
Get expectations in writing
The quote should list surfaces, stain limitations, price, timing and exclusions. Oil, rust, paint and organic growth may require separate treatment. Before-and-after photos are useful for larger commercial jobs.