Question

Why do painters use sandpaper in whitewashing the wall and in polishing the doors

Answer 1

for smoothing the surface area of wall and doors

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.