With all due respect to all my brethren and cistern (?) who have strong opinions on these matters, let me try to summarize some things I think are true:
1) Preparation matters. More than anything else. Particularly for durability. Good prep and the paint will hold up because there is great bonding of each layer to the one underneath. Bad prep and hard finishes lead to chipping, as when wrench blesses a painted surface, stone gets kicked up while actually riding, etc.
2) You can get excellent results (appearance and durability) with a number of different technologies. Stove enamel was ok, lacquer gives better gloss but is much less common. Modern finishes include synthetic enamels and the polyurethanes (of which the most famous is DuPont Imron, which requires absolute attention to industrial hygiene instructions). Powder coat, like spraying, can give shoddy or excellent results, depending on the skill of the artisan.
3) Some folks believe that the original material used matters for proper restoration. That's fine, but might be a frustrating search for material and for painter willing to do it that way.
4) Most of us care much more about matching appearance of the original. To me, that means good color match, proper thickness, and the right gloss. The most common failing, in my limited experience, is refinishing classic bikes with much too glossy a surface. Easy to do with modern paints, and very attractive "curb appeal" for artisan and customer, as well as onlookers.
5) Where this is going scares me: back to the conversations about whether a particular bike should be preserved as a wall queen or be ridden for joy. The occasional chips on the finish of my '38 Paramount after less than a year testify clearly to my preferences: Unless the provenance and story are absolutely compelling, I ride bikes in preference to protecting a museum.
You're welcome to disagree, but I think the underlying issues for most of us are about authentic appearance, and there are few enough about authentic materials that we might be able to put the powder coat argument to bed.
As for me, give me iron phosphate surface prep by someone really obsessive, or show me something better.
harvey sachs
mcLean va