the alloys on my impreza are absolutely filthy on the back - they look as if they have never been taken off and cleaned and you can't clean that bit of the rim on the car due to the brakes

the front faces are mint, it's the inner edge of the rim that you see through the spokes if you know what i mean

i have been scrubbing them with a nail brush and truck clean for 2 days now and it is not shifting the thick black layer of crud on there. i have also tried toilet descaler which works very slowly

have also tried white spirit but it does not touch it

any recomendations on what to use please if anyone has encountered the same problem?