Our earlier corrosion under insulation guide covered why insulated steel fails. This one answers the follow up we received: what do I actually put under the insulation? NACE SP0198 practice organises the answer by operating temperature band, and the shortlist is manageable.
Ambient to around 120 C
High build epoxies and epoxy phenolics rated for CUI service carry this band. Think in immersion terms: the coating must survive wet insulation sitting against warm steel for years. Standard atmospheric epoxies are not automatically qualified; look for explicit CUI or immersion at temperature test data.
120 C to around 200 C
Epoxy phenolics and novolac epoxies stretch into the lower half of this band; above them, inert multipolymeric matrix coatings and other specialised CUI chemistries take over. This band contains the worst of the recognised 50 to 175 C danger zone, and cycling services here deserve the most conservative product choice on the plant.
200 C and above
Inert multipolymeric matrix systems and selected high temperature coatings qualified for insulated cyclic service. Plain silicone aluminium stack paints are for open hot steel; under insulation they need the specific CUI qualification, so read the data sheet line that says insulated service.
Two rules that outrank the shortlist
First, specify for the cycling range, not the nameplate: a line that trips to ambient weekly lives in the danger band even if it runs at 250 C. Second, preparation does not relax with temperature: Sa 2.5 blast and salt limits apply under insulation exactly as they do in a tank.
Send us the line list with operating and cycling temperatures and we return a matched shortlist with data sheets the same working day, via the oil, gas and refinery coatings page.