I had a problem with some original equipment Vauxhall alloys, and I'm sorry to say I can't give you good news.
Over the years I had treatments that were supposed to seal them properly, and paid to have the edges ground to get a proper seal. While each improved things, it never solved the problem and each time it got worse again.
I have heard it said that alloys can become porous, so it may not be the rim where the problem is.
I had to check the pressures regularly and keep topping up.
I eventually solved the problem by selling the car for scrap (OK, it was worn out)
The experience has made me vow not to have alloy wheels again, and certainly not Vauxhall ones.