After the entire guitar is scraped and sanded to a reasonably clean state, I brush it with eggwhite. Eggwhite is mostly protein that marginally soaks in the wood and dries to a hard state. Also the water will “raise the grain” and reveal more of the tiny defects. The next day once dry the guitar is sanded with fine sandpaper and then brushed some more. After 2-3 more repeats the wood becomes much smoother and is ready for the shellac seal coats. This treatment can also give a slightly patinated aspect to the spruce, if done in a certain way.
Later edit: second photo shows the soundboard after a quick first FP session.