Question

why do painters use sandpaper in white washing the walls and in polishing the doors

Answer 1

Likely sanding before painting helps the white wash to stick better to the old surface, by effectively increasing the surface area. It also helps remove any cooking grease and tar from people smoking, which also causes the paint to stick poorly.

Answer 2

Sandpapers provide very high force of friction. Painters rub the sandpaper on the wall and on the door for increasing the roughness of the surface of the wall and the door. When roughness increases, force of friction increases and thus adhesive force also increases. Adhesive force is the force of attraction acting between the molecules of different substances. Paint sticks to the surface of the wall and the door on account of adhesive force.