Just stripped a 75 P bass, it had the thickest sealer I´ve ever seen. Sanding it down would take too much time and it would be a dirty job. I was lucky though, had a beautiful ash body - owned it since ´92 and always thought it was alder.
Guess it would have been natural or sunburst if this lump of wood was one piece, but just under the control cavity you can see the joining of two pieces.
After trying a couple of thinners unsuccessfully, I ended up using a hot air gun carefully with a scraper.
One can always leave the sealer on, if the colour removes easily with thinner, but in this case there was wood repair involved. (Filling a J routing - who would have guessed
)
I´ve seen good results with sanding only, but for myself I get best results not sanding until all paint is removed from the instrument.