How to get rid of black mould stain in sealant of windows

Same as the above answer really, to resolve this problem you would be best to remove all old sealants used both inside and out, then to use an expanding foam from the outside, then to re-seal in low moderate silicon on the outside. With the inside, depending if they have used pvc trims to finish the inside, you will want to use a product called stelmax witch is apvc resin that bonds with plastic and stays white. hope this helps

It sounds to me like you have had your windows sealed in using ether painter’s cork or silicone than has not got an ant-fungal agent in it. This needs to be pulled out and re-sealed using the right sealant. In addition, there may be a leak between through frames so this is work look at as well. Steve Millar