Scrub the surface:
First of all, take a brush and start scrubbing the plaster wall. This is to ensure an even finish.
You must move your hand systematically. Start from one side and keep doing it till the last inch of your wall is covered. Make sure that you are not damaging the pattern of wall. Using a soft-bristled brush is the best way to scrub softly.
Rub a wet cloth on the wall:
After making the surface even, you should remove the dirt from the plaster wall. For this purpose, get a piece of fabric and damp it before rubbing it on the wall. You will see a significant difference after this little exercise.
Prepare the solution:
After you have prepared the wall for cleaning process, it is time to prepare the plaster tri-sodium phosphate (TSP) solution. In this process, you will mix the TSP solution into warm water and pour the solution into buckets. Make sure you are wearing protective glasses and proper gear while mixing this solution. Even a small drop of this mixture can affect your skin badly.
Ensure the ventilation in room:
Next thing you need to do is to ensure the ventilation. Keep all the doors and window open while cleaning the plaster walls. A well-ventilated area is essential to avoid negative effects of TSP solution.
Dip the brush in TSP solution and spread it evenly on the wall. Keep rubbing till you reach the end of that wall. When you feel that the TSP solution has been applied on every single inch of plaster walls, leave the room for couple of hours. This time period will allow the plaster to dry.
Go over the walls again:
After couple of hours, you should apply this solution again and let the walls get dry.