Back in the day, when stove enamel was the usual factory finish, and as a youth, a paintbrush was your only realistic option, it was a coat of Dulux Chromate Primer and a topcoat or two of Black house paint. Those more well heeled enthusiastic perfectionists used Tekaloid Coach Enamel and painted in a dust free environment.....probably the bathroom. So, as an early resto, the yellow is likely to be some sort of primer, still doing a good job
The advent of powder coat changed that. It was a cheap, quick and generally serviceable and consistent industrial finish. Great for clean, new metal. As new technology everyone got on the bandwagon, all with varying standards, were the truth known. The usual way in those days for Mr Average (me) to get his bits done at the cheapest price was by getting it added to someone else's batch for a cash job. This assumed, of course, that all powder coating was to the same high standard. We know better now. Despite assurances, you can bet it went through with the minimum of preparation, and a single thin coat was the result. Plus of course the coating showed every rust pit and imperfection, and you were lucky to get back all the bits, as the typical working environment of that establishment in the back streets of Digbeth, Birmingham in the 1980's, was a bit chaotic.
Powder coatings can appear OK even when the substrate is rusting. As anyone with an Austin Metro or Land Rover with powder coated bumpers usually found out the hard way when they picked a small blister and were able to remove big strips of the finish.
Unless the bike warrants true perfection, modern smooth direct to metal paints will do a good job. Alas, the choice of colour can be limited, as Henry Ford said.
Swarfy.
