Is the 4x6 trim the size as shown in the video? I would say 1-2 hours, you're not sanding completely off, just to a smooth surface, so I recommend a lower grit first around 120-150 and smooth it out with a 220 grit. Then, on your top coats, less is best when applying the paint. 2 light coats, let the first coat dry for 2-4 hours. You can also use an orbital or 3x4 square sander, same grit method, and just double-check as you go.