Update:

The pool consistently looks great. I have been maintaining appropriate balance and have raised CH to 250. The amount of "sand" has steadily decreased. Many days none is visible, although y o u can feel it on the bottom of the pool in spots. The sides feel smoother and very little material comes loose when I rub my hand on it. I'm concluding that a combination of low CH and low PH over an extended period of time softened the finish and caused it to come off. I'm considering taking my CH higher, maybe 300 or 350. I am using cal-hypo but will switch to bleach when I hit my CH target. Any opinions?

FC=7.2
TC=7.2
PH=7.6
TA=80
CH=250
CYA=80