A painted mikvah

Hi,

A mikvah which is painted with an oil-based paint is kosher. Even where the paint is an entity which can receive impurity, it does not disqualify the mikvah, for it is not a receptacle for the water; if the paint were to disappear, the walls would still retain the water.

(Rav Moshe Feinstein, Igrot Moshe Yoreh Deah 1:108)

Have a great day,
Mordechai