Saman,

by increasing the adhesive on the surfaces, but even better  By increasing the surfaces moisture content in relation to the core mosisture content so as to have a surface higher compression ratio and thus higher density. You can also have a higher max pressure of the press (pass from 28 kg/cm2 to 33 or 35), and finally by accelerating the temps of press closing. The best is to combine all these four factors if you can.

Ciao

Tony

@A.Pizzi Pizzi Thank's a lot for your comment. You gave me good idea! I think all four factors that you explained can effect on surface quality.