arunb,
The painting of a carbon steel pressure vessel is not a mandatory activity, but rather a protective measure to prevent the material degradation, when subjected to the environment more or less hostile, corrosive or even erosive.
If the vessel is indoors, in constant temperature and constant humidity, a thin film of a primer paint should last many years (subject of paint manufacturer's specification). Obviously, outdoors, near the ocean, in a salt ladden environment, heavy painting using primer, intermediate coat(s) and finish top coat(s) are recommended. In case of heat traced and insulated surfaces you could face the Corrosion Under Insulation fenomena, where the moisture from the air could condensate over the carbon steel substrate and start a strong corrosion process, usually going undetected for extended perion of time, due to "out of sight = out of mind" . You can prevent this damage by coating the vessel under insulation with one of many paint systems recommended by the paint industry (refer google), perhaps raising to the immersion grade epoxy paints frequently used on O&G plants, also chemical plants. Stick to one manufacturer and ask him to provide the most suitable painting system for your environmental conditions, including the heat tracing and the insulation. He must guarantee the painting system offered and don't ask for any specific advice for use of any paint on this engineering forum, because nobody can guarantee the results of your painting, including your defective application.
Rather, send your email to the nearest paint supplier for advice or better still, to a specialised painting shop, who can guarantee the job.
gr2vessels
