What are condensation stains on aluminium greenhouse profiles?

Condensation stains occur when moisture from the air settles on cold aluminium, for example when your greenhouse profiles are still packed or stacked, or when your greenhouse is already assembled but not sufficiently ventilated. If that moisture has nowhere to go, a greenhouse effect develops, in which white stains (incipient oxidation) appear first and, after prolonged exposure, black stains may follow. A white deposit can still be treated, black stains can no longer be.