How The Foil Test Works
The first step is to choose the right spot. Look for an area where the mark keeps returning, where the paint feels slightly different, or where the wall looks darker than the space around it. Do not soak the area, scrub it aggressively, or cover it with cleaning products first. You want the wall to behave as naturally as possible.
Wipe the surface gently so it is dry to the touch. Then place a square of aluminium foil over the suspicious patch. It should be large enough to cover the mark properly, with a little extra space around the edges. Tape every side down firmly so air cannot easily move underneath it.
Then leave it alone for 24 to 48 hours. Peeking too early or lifting a corner can ruin the result. The foil needs time to trap the conditions around the wall, which is exactly why such a basic object can become surprisingly useful.