Discoloration caused by iron or alkaline substances
Sometimes dark stains may appear on wood surfaces which seem to be caused by mould fungi. However, these stains occur very quickly, whereas mould or blue stain fungi grow gradually.
Dark stains may appear on thermally modified wood surfaces if an alkaline substance is in contact with the surface. Such stains are typically caused by plaster or lime water. Stains can appear e.g. when plastering work is done close to a terrace made of thermally modified wood. In these cases wood surfaces must always be protected from splashes.
Another substance causing discoloration is iron. A typical feature for iron stains is that they appear very soon after rain. Iron can end up on wood surfaces e.g. when terrace surfaces are sanded. Sanding can break down screw or nail heads. Iron then spreads across the entire surface. Discoloration begins when the surface gets wet and causes iron to oxidize.
Also other steel structures close to wood surfaces may cause iron particles ending up on the surface. Water running through rusty gutters or other steel structures may carry iron, which causes stains when ending up on the wood surface.
Reparation of discolored surfaces
Surfaces discolored by iron particles and alkaline substances are a visual disadvantage. When discolorations are removed, the surface must be sanded to such depth that it doesn't contain any impurities. After that the wood surface must be treated. Discolored wall surfaces or railings can also be painted to cover the stains.