Damp keeps coming back on an inside wall that faces outside — why won't repainting fix it?

You've hit the single most common damp mistake in the UAE, and the reason is simple physics.

*Here's the paragraph worth pinning: painting the inside of a wet external wall almost always fails, because the water is coming from outside — so an internal coating just traps it in the wall, and it forces its way back through the paint or reappears at the edges. Water beats paint every time. The fix has to be on the source side: the outside face of the wall.*

On an external wall, "worse after rain" points to water getting through the outer skin — the usual culprits are cracked or blown external render, failed/chalked exterior paint that's no longer shedding water, and — very often — perished sealant around the window frame just above or beside the patch, letting rain track in and show up inside. Rising damp from the base is possible too but less common than a failed outer face or a window perimeter.

The proper fix, in order: find the entry point on the outside (inspect after rain), repair the render cracks, re-seal the window perimeters, and re-coat the external wall with a proper weatherproof/water-repellent exterior system — not just fresh emulsion. Then let the wall dry out fully before you redecorate inside; painting the inside while the wall is still wet is why it keeps bleeding back. It's the same principle as the roof leak that shows in a different spot — seal the entry, not the exit. This is a plastering and damp-proofing crew's job; the internal repaint is only the last step once the outside is watertight.

Worth having someone check the window perimeter sealant specifically, because it's the most-missed cause of exactly this pattern. Over a few UAE summers the external mastic around a window frame goes brittle and cracks, and rain then runs in behind the frame and shows as a damp patch on the wall inside — always worse after rain, never in dry spells, which matches what you're describing. Re-sealing the frame perimeter is cheap and often the whole fix. Rule it out before anyone re-renders half the wall.

Been here, wasted money on three internal repaints exactly like you. The thing that finally worked was accepting the fix was outside, not inside. Once the external face and the window seals were sorted and the wall had properly dried, one internal repaint held and it's been fine since. The "anti-damp" internal paints are a trap — they sell the idea you can fix it from the comfortable side. You can't. What did your painter say was on the outside face?