I can only echo the above comments so I must be doing something right, wire wool and Autosol
When my tips get really bad, I use Cillit Bang Degreaser to get rid of the horrible black stuff- works really well.Finish it off with fine wire wool and Autosol.
I bought my GTI second hand and the exhaust tips are filthy and black,What finish do the tips have from the factory... is it chrome?