Guitar Repair - How do I fix deep scratch in lacquer finish finish

The typical method, if it really is lacquer, is something called a burn-in stick. These rods of solid lacquer come in many colors, to match any finish. To use them, you heat up a flexible, metal palette knife. Use the knife to partly melt and 'smudge off' a bit of the lacquer. While it is warm and pliable, work it into the scratch. Then, level it, sand and buff it as per normal. This kind of repair is often totally invisible.

They stock them at Stewmac:
http://www.stewmac.com/shop/Finishing_s … ticks.html
but lots of high-end woodworking shops carry them too - Woodcrafters, Rockler, Paxton.

You sand using a block, to keep it really flat. That way only the high spot (new material) gets sanded. As you get close to level, you will be wet sanding with fine grit (maybe 1000-1500), so it will dull the surrounding area when you hit it, but only slightly. When you buff the whole area (usually with auto clear coat compound) it will make the new and old match. It is a lot like the mini version of auto body work.