I have been asked a few times recently about getting rid of old damp stains.
Due to this i thought i would share a good way of doing this.
If you do have any old damp staining then you should ensure that the issue that caused the damp has been fixed before proceeding with this next step.
Firstly I would give the area affected a light rub down with a fine grade glass/sand paper, wipe surface with a dry cloth or dust off with a soft brush.
Next with a rad roller, roll a coat of oil based undercoat over the stained area, leave to dry overnight. Depending on how dark the staining was to start you may need to apply a second coat, once this coat is dry apply you required top coat.
For best results coat the whole surface e.g wall, ceiling etc with 2 top coats this will blend the old affected area in with the whole surface area better.
Hope this helps.