You can polish varnish by hand wet-sanding, followed by a bit of polishing compound on a rag. The trick is to work your way up through the grits (probably starting around 320 or 400 grit) and gradually go up through them (320, 400, 600, 800, 1.000 grit etc.) to the finest paper you can find in your local auto supply store (probably something in the 1500-2,000 grit range). The initial sanding is to remove the ding and cover that small area with a very uniform pattern of tiny scratches from the paper. With every new and finer grit, your objective is to eliminate the tiny scratches from the previous grit and replace them with even finer ones. If your final grit is fine enough, a little bit of polishing compound (like 3M Finesse-It) on a soft cotton cloth should put you over the top, gloss-wise.
If you plan on doing this type of thing often, go to the Woodcraft website and buy a package of 5" "Micromesh" sanding disks ($20). It's an assortment running from 1,500 grit up to 12,000 grit, and they will put a shine on almost anything. The 5" disks are made for random orbit sanders, but I use them for hand wet-sanding guitar bodies because that disk size gives you the most sandpaper for your $20. Using them wet both cuts friction and allows the disks to last much longer without the grit pattern filling up and the disk becoming useless. They will last through several projects. You bring the surface up with regular wet or dry paper to about 1000 grit and then take over with the Micromesh disks. Once you have gone all the way up to the 12,000 grit disk, it takes very little hand-buffing to get spectacular gloss.
http://www.woodcraft.com/product/2005232/10574/micromesh-5-sanding-disc-assortment-pack.aspx
