That’s an awesome opportunity! I personally use easy clean from THIS website, you’ll see photos of how it did, I didn’t use anything more than an abrasive pad, steel wool can be used if it’s more stubborn. I personally charge 8$ a pane for hard water removal and I guarantee that 90% will be removed (corners are hard to hit for me). You might want to charge a lower fee because of volume, but it all depends on your area. Good luck!
