After my first airbrush session, I realized that a few spots on the plastic flat surfaces weren't as smooth as non-sanded areas next to the sanded areas.

I had sanded them down from rough to fine grade sandpaper, then run a finishing/polishing sanding file over it to wind things up.

But apparently this didn't work. Any thoughts on what I might have missed? Or general best practices for sanding-to-smooth-finish?

Thanks!

How fine is the fine grade sandpaper you're using? Basically, what grit is it?