I have a bass that I stained with a tung seal finish(appx.3 coats) and followed with a rub-on polyurethane(appx. 6 coats). I have used this bass this way for about 6 mos. The other day I noticed that there were some pick scratches and other abrasions about the body. This didn't sit well. I decided that I would probably end up doing some light sanding and spray on some Stewmacs nitro lacquer. Any drawbacks to using this on a previous polyed surface? And how do I get that super finish on the body like the ones hanging in the guitar megastore?