Mix baking soda in water until well diluted and rub a mixture over the stains. It is important that you do cleaning when it is not raining so that you can open doors and windows to dry more quickly.

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Putting coarse salt in containers in damp rooms will also help finish them off.

Last tip:

It is important to ventilate the rooms in your home and treat humidity at the first signs. Letting it go would only worsen the condition of the wall and endanger your life and that of the people who live with you.