Or if your PH, Alk, calcium got too high, your might have scaling. Sometimes that can seem like pitting.

To fix it, there are a few choices that I know about:

1) Replace plaster. $$$$
2) Drain and acid wash. Unless you are daring, it requres a professional.
3) Lower PH and alkalinity and brush brush brush. This much like an acid start of a new pool. The idea is similar to 2 but does not require a drain but does require a lot of acid and baking soda. You drop the alkalinity to zero for a few days and brush the pool three to four times a day. Then you add the baking soda to rebalance the pool. Note that this method like #2 will reduce the surface life somewhat but it is much less expensive than #1 or even #2.