Hi Hamish!
Not ignoring you...just still giving it some thought.
Looking back through the Forum archives and searching the 'Net, I see two possibilities:
1) Some sort of very resistant surface contamination. Amazingly, I have found some things you'd imagine to be lubricants to actually cause sticky spots on rims. For me one time, it was diesel droplets dropped by a rich-running truck I was following. I had the hardest time removing it, and the rims weren't the worst of it. Got my clothes, my glasses, the paintwork on the bike, everything. I finally broke out the Big Guns to get it off the rims: An old bottle of 1-1-1 Trichloroethane (
http://en.wikipedia.org/wiki/1,1,1-Trichloroethane ). If I ever have kids and there's hands growing out of their foreheads, you'll know what caused it; this is nasty stuff. Two sets of nitrile gloves and a respirator and I still thought I could smell it (vaporous fumes probably absorbed through my skin). The point is, some rim surface contamination can be very, very stubborn to remove. I have had good luck in some cases using automotive rubbing compound. The clays in it absorb contaminants and leave the surface even and clean, but might actually stick in the tiny voids of your CSS rim treatment, so use with care if you give it a go.
2) Perhaps the rims have begun to wear through at this one point, Hamish, or are worn "thinner" there. I would think the substrate would need to show wear overall before this happened, but I have seen it occur occasionally in isolated spots on Mavic's ceramic rims (which, of course, use a much different process than Rigida do with CSS). In any case, if the coating were compromised in some way, perhaps the alu substrate would react with a different coefficient of friction in relation to the pads. It's a thought, however unlikely.
I'll keep my eyes open for a more definite cause and -- hopefully -- solution. Meanwhile, this is the best I have at the moment.
I hope something in this will help.
Best,
Dan.
