Based on some of the work I've done at the Bike Exchange I'd say longevity of the paint/finish can be a challenge for very small builders who don't have or can't access the capitol equipment required for chemical dip-type surface treatment and prep before paint. I rebuilt a Boone McReynolds frame recently that had a lot of touch-up already performed and rust bubbles appearing here and there (not just on the top tube where sweat falls). It looked to me like Imron paint that had been applied perhaps even directly on the bare metal - I couldn't find evidence of a phosphate coating or primer where the paint was coming up. The paint adhesion just wasn't very good. It's a shame too, because all of the skilled work at the brazed joints and the thinning of lugs was otherwise exemplary. As you mention with respect the Merz frames, not all small builders have this issue - I rebuilt an Ed Litton bike of a similar age about a year earlier and the finish on that bike was far more robust and consequently in much better shape.