Hard water spots are classified into two categories. Type I are surface-level mineral deposits. Type II Water Spots, however, are sub-surface etchings. When "hard" water (rich in calcium and magnesium) dries on a hot panel, the water evaporates, leaving concentrated minerals behind. If exposed to sunlight, these minerals become alkaline-active and chemically "eat" into the clear coat or glass. This creates a permanent, concave crater that cannot be washed away with soap; it requires mechanical leveling of the substrate to restore optical clarity.