You mean the small gaps in the filament?

Those won’t go away with sanding unless you go really deep

And personally for a prototype sanding is a waste of time. Get something working as you expect, then roughly design a case, iterate the case, finalize the case design, then finally worry about surface finish.

What the other person said. This is a prototype and looks to be printed with thicker layers than normal (.2mm is the usual.) to save time.