Question

Why do painters use sand papers in polishing doors ?

Answer 1

The flawless gleam of an oak floor, the smoothness of a painted wall or ceiling, the high shine of a varnished tabletop — all are signs of a job done well. And all are made possible by the patient, methodical application of sandpaper.

It's a task many of us wish we could avoid or cut short. Even This Old House master carpenter Norm Abram.

"Pushing sandpaper often seems like an endless job," he says. But without proper sanding, any imperfections will be magnified when the final finish coat is applied.

The secret to getting good results with less tedium is to choose the right paper for the job. Sandpaper works by scratching away defects with thousands of tiny abrasive particles. The bigger the particle, or grit, the bigger the scratches. Hence the iron rule of sanding: Start with a grit coarse enough to quickly remove surface imperfections and follow with incrementally finer grits. Each successive grit erases the scratches of the coarser one before, until the scratches themselves become undetectable to the eye and the touch. To prepare bare wood for paint, for instance, Norm starts with 80-grit paper, followed by 100, 120, 150, and 180, and finishes up with 220. (With most sandpapers, the coarser the grit, the smaller the number.) The temptation is to skip a grit in the sequence. "People get impatient," Norm says. "They think a surface feels okay, but after the finish goes on, every flaw and scratch pops out."
I hope it will helps

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.