I tool one of the EPA mandated spray gun classes put on by SATA a few years back. In many cases the MFR recommended pressure settings are less about optimal performance and more about meeting a federal regulation. You may find bumping the pressure slightly beyond what is recommended will give a smoother finish to what may appear as a slight orange peel. I'd also say this would not apply as a fix to your sandpaper finish, first guess would be that your fluid tip is not tight enough to the housing or the seal needs replacing..
