Wheel rims leak for several reasons. Steel rims leak primarily because of rust that accumulates between the tyre and the rim. Water collects there and rust is inevitable. Aluminium rims don't rust, but are not immune to leaks. The tyre rubber, over time, begins to bond with the aluminium. When a new tyre is mounted, a rubber-to-rubber seal is created that can more easily leak than rubber to aluminium.
Remove the valve stem and let the air out of the tyre. Use a bead breaker and sledge hammer to break the bead. It is not necessary to remove the tyre, however, it may be easier to clean the rim if the rust or contamination is extensive by doing so.
Clean the rim with a wire brush or sandpaper. Remove the rust or rubber residue. The object should have a clean, level mounting surface as much as possible. Inspect the rim for damage or dents. Bent rims must be professionally straightened and checked for run-out.
Inflate the tyre and install the valve stem. Use a spray bottle filled with soap and water and spray around the entire rim. Any leaks will begin to bubble. Verify the repair and install the tyre.