Before Low-E or mirror coating process, glass should be extremely clean.
For standard washing machines with roll brushes, the glass pass quickly. It’s good and clean enough for most processes. The washing machine for coating needs a special flat brushes, they have a larger contacting area with the glass compared the roll brushes.
Normally polishing powder will be added in the flat brush zone so glass can be cleaned even better. So the water in this zone will be “dirty” and it’s better to stay in this zone, or else it will contaminate the water in the roll brush washing zone.
After polishing by flat brushes, roll brushes will wash the glass like standard process. But in the final washing zone, DI water should be used for final cleaning.