Review on Drill Brush Ultimate Automotive Cleaning Kit with Extension: Accessorize Your Truck with Ease - Clean Glass, Upholstery, Seats, Windows, Interior, Wheels, and Carpets - Includes Car Mats and Spin Brush for Efficient Cleaning - Perfect for Motorcycle Accessories by Noah Smith

We used these brushes to recently clean a 20 x 20 foot slate patio. I used an electric drill and wheel brushes to remove the excess grout. They worked really well and I basically just used one brush for all of this (one tile at a time). It lasted all day. At the end of the day, the brush is worn out but still functional. I love them and I'm sure they will last a long time. The brushes used were "black" hard.