Stagecoach 12 gauge recrowning?

First suggestion would be to pick up a good book on gunsmithing. If you like to work with guns you will pick up some great ideas. As for sandpaper, wet & dry with a grit no larger than 600 might work. You don't want to do something too harsh or you leave ugly marks on the barrel. You can buy cold blue material for touch up work...but it might not match the rest of the barrel. One thing you can do if you have a good buffing wheel is to buff the entire set of barrels and apply cold blueing to the full set of barrels...at least that will make them all the same color. If you really get into the project you can do hot blueing but that is messy and time consuming. And you still have to polish the barrels. Get a book, however, and follow some instructions for cleaning up the barrels. The gun is a good gun, possibly made by Rossi and sold first around 20 years ago, As for the sloppy varnish, you can also re-finish the gunstock and if you get ambitious, you can add some checkering. This could be a great project if you get into the idea.