Eh, yes and no to the truthiness. You can polish some woodworking finishes, but I don't count polyurethane among the ones you can. It's too soft to take a good polish, generally the finish off the brush/spray is the best poly looks. I've tried before, and I've never once managed to get a good look polishing poly.

If you're determined to try though, the first thing that has to be done is the finish needs to be fully cured. For an oil based poly, that generally takes a month or so. After that, start with 320 grit wet/dry paper, sand wet and work your way up through the grits, 320-400-800-1000-2000. Rubbing out a finish I go up to 2000, the move to automotive rubbing compound, then automotive polishing compound to get a mirror finish. Generally though I only rub out shellac, as I've found it takes a nice polish

 

Before doing anything let the finish cure a month. The polish alone won't do it. If the finish is reasonably smooth start with 1500 grit or finer and wet sand the finish. If not then use epicfails schedule of sanding but each time you go finer spend more time with it. Then use rubbing compound with a 7" auto polisher with a lambswool bonnet on it. When you think you have the gloss restored try the clearcoat safe polish and see if it looks good. If it still looks a little dull start over with the rubbing compound.