Well (knock on a wood) it seems paint job is not bad. On other hand there is some rust inside of a tube already (after just taking out of a box for first time), especially in a longer side part. In fact I am quite surprised that inside is not in any way treated with anything.
It has been quoted countless times, now it has been quotet countless +1:
DURABILITY
Before painting, each frame is pre-treated with a chemical anti-corrosion treatment. This protects the bare metal and prevents any surface corrosion that may develop, penetrating through the metal. Often visible in the open tube ends, this rusting is on the inside of the frame tubes and is purely cosmetic than a structural problem. The electrostatic spraying of powder paint means the powder cannot enter into the inside of tubes easily and means that there is no paint on the inside of tubes beyond the first 3-10mm. Consequently, the inside surface of the frame on a Brompton is unpainted and can appear ‘rusty’ but this will only be a surface discolouration due to the chemical anti-corrosion treatment.
As the steelwork is protected from corrosion there is no need to apply any rustproofing or similar to seal the tubes. Blocking the ends of the tube can do more harm than good and can actually trap moisture inside the frame and not allow it to breathe. Wiping with an oiled rag will lessen the appearance of this yellow-red colouration.
This quote is from
Brompton's support pages. It is a good idea to check i.e. there or to
ask before making claims like your "that inside is not in any way treated with anything" - which is simply untrue. Such wrong claims are the reason why forums are often a dubious source of information and due have a bad reputation in that aspect.
