Concrete block is a porous material and must be waterproofed to prevent seepage. Moisture seeps gradually through concrete and causes mould growth and deteriorates concrete and mortar. Building codes require builders to waterproof exterior concrete block below grade (ground level), but local code varies greatly in enforcement and minimum requirements. Additionally, code says nothing about interior concrete surfaces. To be safe, have your subgrade exterior wall professionally waterproofed and seal the interior yourself to ensure a watertight wall.
Scrub the surface with a mixture of warm water and detergent to remove dirt and debris. Dry with towels.
Disinfect the wall with a mixture of household bleach and water (1/2 a cup of bleach per gallon of water) to prevent mould growth. Allow the wall to air dry.
Apply commercially available concrete sealer to interior surface using a roller, paint brush or sprayer. Apply sealer liberally and evenly, moving from top to bottom in long, even strokes. Completely cover one wall before moving to the next.
Allow sealer to dry. Drying time will vary according to environmental conditions such as room temperature and humidity; check after an hour or so. Apply a second coat, and allow it to dry. The second coat should take less drying time than the first coat.
Most block foundations are waterproofed by the builder during construction. Check periodically for signs of moisture seepage, such as musty odour, mould growth and dampness.