I had the ones on my First Edition replaced about 6 months ago, the corrosion was around the outer edge of the wheel and on the edge of some of the spokes, it was like little white flecks on the silver of the alloy
It's the laquer peeling or flaking off. Which causes more corrosion and more laquer to peel off. Was a notorious problem on a lot of VW alloys. Luckily under warranty. Problems worse in winter apparently when the coating is less flexible and more brittle, not to mention the damp conditions. Hope you get them sorted.
Do you not think some of these alloy wheel cleaners are way too strong and can cause oxidisation if there is any break in the laquer coating. Most of these cleaners are a very strong alkaline and that stuff loves bare alloy to get its teeth into. If you wash your wheels regularly with normal car shampoo and wax them after, it will not allow the brake dust and road grime to stick to them.
I agree prior to painting alloys you have to use an etching primer or the paint will flake off within weeks.
Company I work for has just gone bust so I have plenty of time in my hands... spent the last 2 days washing & polishing the car, but as Techno said best thing is warm soapy water & then I wipe LPS 1 or 2 on them....one & half winters so far and no signs of corrosion
the acid cleaners (if you can still buy them) are definitely best left alone. and wheel cleaning products from car accessory shops preity expensive and i find no better than the spray on bathroom + kitchen cleaners mr muscle, flash, etc, which are much cheaper, but better at getting the brake dust of than soap and water.