Mold is a common problem in any environment that is perpetually damp, especially if there is a lack of sunlight in the area. Mold can strike anywhere for this reason. While it is not difficult to remove in small amounts, it is a real challenge to remove mould from paintings. Oil paintings require careful cleaning, but watercolours even more so, as exposure to moisture can cause their delicate colours to run. However, with proper care mould can be removed from watercolour paintings.
Brush at the mould with a soft-bristle brush. The light friction can be enough to remove some or all of the mildew without affecting the paint.
Place the painting in the sunlight during the day for several days. Direct sunlight kills mould and can remove it from paintings. Sunlight fades paint over time, but several day's worth of intermittent exposure will not cause any damage to a watercolour.
Wipe at the mould with a small piece of light-grit sandpaper to carefully remove the mould, if the other methods are not working. Be very careful when doing this because the sandpaper can potentially damage the paint if too much force is applied. Dust the painting afterward to remove any loose sand that has come off the sandpaper.
Have any valuable or sentimental paintings professionally cleaned or restored to decrease the odds of damage.