Why do water spots remain after washing my car?

Washing removes loose dirt but may not dissolve mineral deposits that have bonded to the paint. If spots remain after washing, they are likely mineral residue or etching rather than ordinary dirt. Try a dedicated water-spot remover on a cool panel first; if the outline is still visible afterward, the spot has etched into the surface and controlled polishing may be needed depending on its depth. Old, layered deposits may need several careful applications, and deep etching may not be fully correctable.