The black on the back of the neck was also very thick. I completely removed the finish on the back of the neck, down to the rock-hard filler. So I worked slowly and patiently, drying the wood often and checking what it looked like. As soon as it starts to take on color, you are through the clear coat and are now removing colored lacquer (or poly, whatever). As long as it is milky white, you are removing the thick poly clear coat.
To be more complete, when you wet sand, you will produce a milky white sludge.
