The Golf's wheels with gloss black and polished (Austins, Parkers etc) aren't that easy to clean well. The 18's are tight to the larger brakes at the front and black gloss is very easily swirled and also brake dust likes to eat it. I use wheel woollies and the middle size brush works on the front and larger one on the rears. Using a fall out remover first helps (Bilt Hamber Auto Wheel or Korrosol). I used Platinum Wheels on mine after 12 months/8k miles and it was extremely hard work to get them properly clean which included full clean, clay bar, hand polish then sealant.
A ceramic sealant will make life easier but once a wheel is past it's best, it is extremely hard to get it properly clean. But if it's had a life of cheap hand car washes (harmful products) or lots of miles and not a lot of cleans, sometimes a full refurb is the only option if you want mint wheels.