Impurities on painted surfaces with “black paths” on vertical exterior surfaces.
- Usually vertical surfaces, where rainwater “licks” on them along with air pollutants (mainly soot, smudges from fireplaces, radiators, grills, etc.).
“Hydrophobic or waterproof” surfaces that do not allow rainwater to remove dirt.
Construction failures (eg lack of marble barrier).
Good surface washing, priming.