How old can the pool be and still get an acid wash?

Age doesn't really matter, as long as the pool is gunite or concrete. But never acid wash a fiberglass pool, or a fiberglass lining in a gunite or concrete pool. I suppose I should explain why, because so few pool pro's understand fiberglass, you will probably receive considerable conflicting information.

The finish coat on all fiberglass pools manufactured in a factory is swimming pool gel coat. Unfortunately, this is not true of all fiberglass linings in gunite and concrete pools, although it should be. Gel coat is non-porous which is why it's such a fantastic finish, at least for the first 18-20 years, after which it becomes porous and must be removed and replaced.

Using muriatic acid on non-porous gel coat will cause it to become porous, at which time it becomes worthless, and must be removed then replaced with new gel coat. Once it's replaced, you're good for another 18-20 years, unless muriatic acid is again used. -Bill