The particular method for cleaning interior concrete walls depends on whether the walls are being primed for painting, old paint is being removed or mold is an issue. Concrete is porous, so it retains moisture, and this can affect the integrity of paint that is applied following a cleaning.
Basic cleaning to remove dirt or mold involves using a solution of four parts water to one part bleach, according to the home maintenance website How to Clean Stuff. Use a scrub brush to completely remove dirt and mold. If the grime is grease-based, use a de-greaser in place of bleach. Because of concrete's porous nature, cracks need to be patched and the walls sealed to avoid mold growth. If there is no paint on the walls, they can be painted after being sealed.
If the walls were previously painted, the old paint needs to be cleaned off. This can be done manually with a wire brush or a paint scraper. This method is time-consuming and labor-intensive. Alternatively, use a shotblast machine. This machine shoots tiny metal beads at the wall to remove the old paint. Another option is using chemical paint remover. This should not be done unless the room has proper ventilation, as the fumes are toxic.
After sealing the concrete, the walls should be primed if they're getting painted. Concrete primer evens out the surface by filling in the pores. Wait at least eight hours before painting. Use masonry paint because it binds to the surface, meaning it will expand and contract with the concrete to prevent future cracking.