Water spots may appear as rings, cloudy patches, mineral deposits, or local surface texture. They can form before heating from droplets on the sheet or after forming from poorly controlled cooling spray.
A cold sheet moved into a warm humid area can collect condensation. Compressed air and cooling water may also carry moisture or dissolved minerals. On some materials, a wet surface during heating can create a deeper optical defect than a removable deposit.
Corrective action
The timing of the mark should be established by inspecting the sheet before heating and the part before cooling spray. Storage and staging conditions should prevent condensation. Air systems need effective moisture separation, and spray nozzles should produce a controlled pattern that does not flood appearance surfaces.
When water cooling is used, water quality and drying must be managed. Wiping mineral residue from finished parts does not correct a process that repeatedly deposits it.