<p>Thanks everyone, it makes sense that it's some form of solvent, I just can't figure out what. Unlike my more utilitarian lenses, I've treated this baby like it's porcelain. Trying to think if I ever took the lens to India, which is the only place I really use insect repellant. Also, it seems evenly distributed across the whole rubberised paint, not just where my hands have been. Anyway, it's not a huge problem, just leaves my hands black and sticky, like touching fresh tarmac on a hot day. I'll pop by Canon, and see if there's anything that can be done.</p>