Traditional glass and mirror cleaners contain IPA,ammonia,solvents,etc, and can leave a greasy film behind on the glass which is usually very noticeable on sunny days, we find magic eraser pads dunked in a bucket of warm water very effective at removing these residues, if you are cleaning many windows make sure you change the water after every few windows, then just finish off with your wfp or trad system, of course you will be charging extra for deep cleaning these windows, for anyone who has not tried magic eraser pads to deep clean glass they really are an amazing tool to have as no wfp (hot or cold) or trad system will deep clean glass like these little fellows.