Mould grows in conditions of high humidity, poor ventilation and on surfaces with a high moisture content. Condensation or poor air circulation is the main culprit in most cases. Mould can be very destructive to paint coatings and needs to be eradicated before applying paint. Treat affected areas with a fungicidal wash or similar. Leave for 24 hours, wash down to remove residues and allow to dry thoroughly. If necessary, repeat the treatment. Improving the ventilation and the use of mould inhibiting paints will reduce further contamination.