Question

smoothing of surfaces beyond the limit increases friction ..... why ?????

Answer 1

Smoothing of surfaces actually reduces the surface.

Actually, smooth surfaces don't allow the interlocking of two surface.

It just decreases the adhesion of force.

Answer 2

This is because on smoothening the surface beyond limit the surface area of contact increases,the surface molecules are able to come close to each other as a result the electrostatic force(adhesion) between the atoms of the surface increases. Therefore friction also increases.