I am using minwax water-based wipe-on polyurethane in clear gloss. At 9 coats, it's pretty shiny, and remarkably flat. That's why I stopped here with the ukelele - it seems done. But when I wet-sanded it, I sanded through in places, even though I felt like I was barely touching the surface. That's why I was figuring on double the coats of the ukelele. But I don't know what the signs are that you're done, since I obviously stopped too early the first time. The ukelele polished very nicely, but it's a 20$ kit going to a 6 yr old, not quite the same as guitar that's hopefully going to be used and treasured for years. I really want the finish to hold up.