The phosphate finish itself is actually very smooth, but the military required the pistols to be sandblasted before phosphate, and this creates the rough finish.

On the late Colt 1911A1's the Parko-Lubrite was so smooth that Ordnance let Colt discontinue the final polish of the feed ramp. The outside was sandblasted, but the top of the receiver was not, leaving the outside slightly rough, where the inside was very smooth. Also, the slides of most of the late Colts were not sandblasted, and they too are very smooth. This led some to believe the receiver was Parkerize and the slide Parko-Lubrite, but both are Parko-Lubrite.

This is a late Colt with the sandblasted receiver and non-sandblasted slide. Same finish, just one sandblasted and the other wasn't.