Since the topic of painting bikes came up a few days ago I had the occassion to observe a few things on a frame that just came in. This frame had a recent professional paint job on it and it brought to mind a few things that would be helpful for you do-it-yourselfers to know about. These points are particularly important on earlier frames where there are few or no braze-ons for cable guides (both tt and bb), shifters, front der., and pump pegs.
The bike had had parts clamped on but I don't think it ever got on the road after the repaint, or if so only for a very short time. In each spot where the clamps were located, the paint had "torn" away from the primer as the heavy coats of paint and clear were squeezed out from under them. Rust would have begun forming as soon as the bike would have been used. As Dave Feldman mentioned, the seat bolt and seat post had also caused chipping in addition to anywhere the wheels had been and where the freewheel had touched the chainstay. None of these things should happen to a paint job, wheather new or old. The cause of these problems were three things, which I will explain.
First, in an effort to save a step, white primer was used. My experience has been that white primer not only doesn't stick to the bare metal as well as the grey-green formulation of the same stuff, but paint doesn't bond to it as well either. It's better to use the green primer and lay a coat of white down before the color than to use white primer.
Secondly, the primer was applied way too thick. These types of primer are formulated to go on in one thin coat, sometimes almost so you can see through it. That is what gives the foundation of the paintjob its strength. Heavy primer coats are very weak and contribute to "thick looking" results by the time the painting is done. The rust inhibiting properties are fully intact, whereas if the paint comes off of it the primer won't prevent rust without paint on it.
Third, as we discussed before, the paint layers were also too thick to the point that where the clamps were, one could have poured plaster in it and made a perfect casting of the clamp since the "walls" created by the paint smooshing out were so high. All of these technical aspects are important to attend to since a beautiful paint job is of no value if it doesn't hold up past the point when one starts hanging parts on the frame.
So keep the primer thin and continue to the end that way if you want a durable finish. Thick primer may work on a classic auto with a hand-rubbed lacquer finish; but it's a recipe for failier on a bicycle. Hope this information will be of some use to you guys who like to work on your own stuff.
Brian Baylis La Mesa, CA Damn, there was a bicycle under all that stuff!