Although concrete can be made virtually watertight, it remains an inherently porous material. Micropores cannot be avoided and small hairline cracks will develop over time. This is part of the long term curing process and is the nature of concrete. Micropores and hairline cracks that allow some moisture through can add to the charm of concrete by allowing some moss growth and discolouration.
Planters that are ordered or purchased unpainted are not treated outside nor inside. All painted pots are given a water-based waterproofing coat inside. Although this greatly aids waterproofing, planters cannot be regarded as water containers and slight moisture penetration is still a possibility.