my alloys are now a few years old, they are a multi spoke alloy, there is a build up of black grime/brake dust between the spokes. no amount of power hosing will remove it, any readers have any info on products/methods that may help
Depends how fussy you are maybe but I've found that zipwax on a car sponge, individually rubbed on each spoke (rinse/repeat) works fine on my 5 year old alloys.
You can buy alloy wheel cleaner in Halfords, Atlantic etc. Spray on, leave for a couple of minutes (no more or it could damage the finish) rub with a cloth \ sponge and rinse really well. Once wheel is clean, I apply a layer of car wax polish, this protects the alloy and makes cleaing really easy next time. Bear in mind that brake dust is corrosive and will damage an alloy over time if left uncleaned.