Quoted from MustangPaul:
Aint that the truth. I wouldn't be surprised if he needed another 5 gallons. Then most likely 2 coats of paint.
Yup, raw drywall and compound is a sponge. I'm guessing he'll need at least another 10gal. I'm surprised that 10gal would even be enough given the size of his space.
P.S. Following this project - green with envy.