Can hard water damage a ceramic coating?

Yes. Hard water leaves calcium and magnesium deposits on top of a coating when droplets evaporate, and if this happens repeatedly the deposits can build into a film that is difficult to remove and suppresses water beading. Severe mineral spotting can also etch the coating, or the clear coat beneath it, especially when combined with heat and sunlight. Drying the vehicle promptly, using filtered or softened rinse water where possible and periodically using a coating-safe mineral remover reduce the risk. Once a spot has become a physical depression rather than a deposit, chemical removers cannot fix it; the area usually needs polishing and then recoating.