The thing is - you need to know your brushstrokes. Not the ‘meh I don’t know what I’m doing’ brushstrokes - not ‘these are just the unimportant brushstrokes til I put the important ones on top’ but EVERY brushstroke. The very first to the very last are all part of the same rhythm.
There is no underpainting. The underpainting IS the painting. The overpainting is the painting. It is all the painting. It is just the painting.