Steve mentions the thin plastic shims; I used to use sections of inner tubes for my rack and generater and such way back when. But they'd be way too thick for classy classic stuff such as Campy downtube shifter clamps.
But what about something thinner than an inner tube? Trying to think of such a thing, I hit on the idea of maybe using a section of a condom (non-lubricated, of course!) between clamps and tube. Don't know if it'd work; just thought of it. And maybe DF is right: if it ain't clamped directly to the tube, it'll slip. . . .
But maybe the condom is worth a try as a paint-saving prophylactic. Does anyone out there know from experience? Or am I thinking crazy things, driven mad by the snow?
As for the French tourers and the brazing, I think DF is right there!
nath dresser spring green, wi (where March has pounced like a lion)