Excessive rain penetration can cause damp patches on walls the moisture attracts dirt from the atmosphere so that the wall appears grimy in addition mould can then develop and green algae can grow if the dampness penetrates inside then patches of mould can form on the interior walls creating a damp and musty smell by treating the outside wall with hg hagesan water seal water can no longer penetrate the wall so problems caused by excessive moisture are unlikely to occur hg water seal has