Back in the day we would come across an aluminum wheel that was porous and the fix was to remove the tire, wipe the wheel with alcohol, and and apply a thin coat of latex pain to the inside of the rim. Haven't seen one like that this century though & all cars are on factory aluminum wheels (not necessarily ones that came on them though).

Check the Crossie about monthly but think I only added air once in the year I've had mine.

If having an issue, I'd dismount the wheel and spray the rim where the tire meets the rim (bead area) with soapy water and watch for bubbles. If that is the problem most tire shops will have a can of bead sealer that may be what you need. Tire does need to be dismounted to apply.