Wet-sanding

You should anticipate that it will take about four or five applications of three coats each (and subsequent wet-sandings) before the parts are finished.

Source: wiktionary

When your primer coat is dry, it's on to the wet-sanding to get that paint flat and silky smooth.

Source: wiktionary

The stock pores should be filled by this 400-grit wet-sanding with just a trace of finish left on the surface. One more wet-sanding with 600-grit paper is done, leaving almost no finish on the surface.

Source: wiktionary

Wet-sanding with a damp sponge works well too.

Source: wiktionary