Rising damp, that dark tide-mark that climbs from the base of a wall and doesn't go away even when painted over, has a specific origin and needs a specific treatment different from that of a localised leak.
Why it rises and why painting doesn't affect it
Ground water climbs through the pores of the building material (brick, stone, old mortar) by capillary action, much as a sponge absorbs water from below. Painting or tiling over it doesn't interrupt that process: the damp is still there and usually reappears more strongly at a nearby point.
Injected chemical barrier
The most usual treatment consists of drilling the wall at a set height and injecting a water-repellent product that creates a waterproof chemical barrier within the material itself, cutting the capillary rise of water at that point. It is a treatment that requires specific machinery and curing time.
Why the subsequent coating matters as much as the treatment
After treating the cause, a macroporous (breathable) mortar coating is recommended in the affected area, which lets the residual moisture still in the wall escape without trapping it behind a conventional render, which would cause new stains or detachments.
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