In the DIY spirit, I cut a doughnut shaped template which left the inner edges of the disk, that were not swept by the brake pads, and the bell area exposed and treated to some VHT paint which worked very well. Even where a little paint got on to the swept area, it was quite a powdery effect to the paint which simply got pushed away after a few brake applications to leave a nice round edge....this was without machining away any metal work.
This was on untreated discs. I had less success with Pagid discs which already have a grey protective coating - the VHT didn't stick so well and came off with pressure washing.
Either way, both had a better end result that the £200 or so that I spent on pre-treated Black Diamond grooved discs which was showing clear signs of rust showing through after only 2 weeks!! So much for the black phosphate coat that they used supposedly being rust proof - they were off the car within 6 weeks as they looked so bad. In my experience, forget the fancy stuff, save the money and treat the discs yourself.
