Use a pumice stone for calcium scale — carefully.

The best way is a pumice stone, used wet and gently, along with a scale-specific chemical.

Calcium scale is hard, but pumice is softer than plaster, so it won’t scratch if you keep it wet and don’t press hard. Rub lightly in small circles. If you’re heavy-handed, you’ll still leave marks. For bigger areas, try a scale remover with phosphoric or dilute muriatic acid — follow the label to the letter and neutralize afterward. Pitting usually comes from concentrated acid sitting too long or using a wire brush. Avoid both of those.

Patience is cheaper than replastering.