When a tyre is leaking, spotting the origin can be difficult. If you determine that the tyre is undamaged but air is still escaping, there is probably a leak between the tyre and the rim. This can occur when water seeps into the wheel and rests where the tyre meets the rim, called the bead seal area. The metal rim becomes corroded and an air leak is created. This occurs most often with aluminium or alloy rims.
Take the wheel off of the vehicle. If there is any air left in the tyre, remove it by pressing down on the valve stem. The bead seal area should now be exposed on both the tyre and the rim. It is not necessary to remove the tyre from the rim.
Remove the corroded areas on the rim. Run a buffing wheel made for removing corrosion along the bead seal, polishing the rim back to a smooth surface.
Wipe the area of the tyre that touches the rim with a solvent appropriate for rubber, to clean dirt off the tyre. This helps create a better seal when the tyre is inflated.
Apply a layer of tyre glue to the rim's bead seal with the tip of your finger or a cotton swab. Though this is not required, it can inhibit further corrosion, by preventing moisture from settling in the rim again.
Inflate the tyre and remount on the vehicle.