With a really stubborn leak, I like to use a water tank, child’s wading pool, local pond, bathtub, or anywhere you can submerge the tire just above the rim (about 4 or 5 inches deep). The spray bottle works most of the time, but with a really slow leak, you may be unsuccessful. Don't forget to check (submerge) the tire valve. If the bubbles exit from the rim area, you'll need to remove the tire. I have often discovered where the owner or some careless shop has gouged the rim (under the tire's bead) when using a tire tool or (heaven forbid) a screwdriver. I have even discovered rims that were ruined by dozens of deep gouges. Minor scratches can be sanded out with care.