papasmurf

Corrosion on the face where the tyre sits on the rim is in my experience the main reason for slow air pressure loss.
It is easy to determine this by spraying a strong mix of washing up liquid and water on the join of the tyre and rim. If bubbles appear you need to mark the spots, take the tyre off and polish the face where the leaks are occuring.
(It may be possible on SOME bikes to quite legally put a tube in the wheel if the problem persists.)