Not so much a babe, it took me 15 years before I tried it for the first time.
Still plenty to learn though, especially when it comes to instruments.
Yes, if I sand through I apply another coat and try again. The instruments I've built have actually all been done with nitrocellulose lacquer. I started doing rubbed out polyurethane on pieces where I needed much better protection (e.g. tabletops) but didn't want the plastic look of polyurethane off the brush. I use 0000 steel wool for relatively matte finishes (followed by wax), pumice, rottenstone and automotive buffing compounds for more sheen. Working by hand and relatively gently I haven't experienced polishing through layers. With sandpaper yes.