what did you do with your fazer today ?

[Grahamm]

Next time you, clean off the baked on dust as you call it, put the piston under a strong magnifying lens.  What you'll find is a tiny pit in the cadmium, the dust is clinging to corrosion, as the coating is damaged. 

The crud on mine was definitely on the surface as I checked by using the "clip" from a plastic biro cap to try and scrape it off and it was catching on the crud when I ran it across the piston.

In any case, it's outside the caliper body when the pads are in place, so it's not going to damage the seals. It probably would be even if I put new pads in, but they don't need replacing at the moment.