Dear Ms Anna
To find the best solution, first you need to understand the problem. There are 3 types of damp and all 3 have a number of different ways to deal with them. Plastering over it is not one of them....Covering up a problem will not get rid of it and damp is like garden weed you need to kill it in the roots otherwise the problem will be coming back and please do keep in mind damp can be very bad for you health, even worse if the are children around.
Bellow are some DIY tips on how to prevent condensation and damp in you home.
1) Check the pipes, drainage and gutters for any blockages including moss, leaves, dead birds etc. Remove the blockage and you might find that the damp problem goes away easily!
2) If the guttering is old, it might need to be replaced
3) Ensure that any cracks in the walls or window frames are fixed
4) Have a good look at the roof. Are there any loose slates? Problems with the roof are a common cause of penetrating damp problems
5) Check that all the windows in the property are opened regularly to allow a ‘change of air’
6) Use extractor fans in the kitchen and bathrooms – rooms where there is a lot of steam
7) When using the kitchen or bathroom keep the doors shut and extractor fan on so that excessive moisture does not go into other parts of the house
8) A climate control humidifier might help
9) Ensure washing machines and dryers are adequately plumbed in according to the manufacturers instructions. Incorrect installation can lead to steam escaping
If you need any more assistance, do not hesitate to contact me.