Eugene, OR
Hard water stains -- usually sprinkler overspray or mineral deposits -- need a professional-grade compound made specifically for glass. Standard glass cleaner won't fully remove them; it's offered as an add-on to a regular cleaning for an additional charge.
The minerals in hard water etch into the glass surface over time rather than just sitting on top of it, which is why wiping with a regular cleaner only removes part of the problem.
If sprinklers are the cause, redirecting them away from the glass is the real fix -- otherwise the stains will keep coming back after each treatment.
