I guess mud would be fine, but you have to get it on the inside of the headers as well to be really efficient.
Ceramic coating in WA is done by CIC high performance ceramics. They quote,
"Thermal protection. The coating is a thermal barrier, thus keeping heat within the manifold or header. There are a number of benefits for this. First, by keeping heat within the manifold, you’re going to accelerate the exhaust gas velocity which reduces back pressure and reduces fuel contamination due to reversion. This is a performance benefit. Second, you’ll reduce the surface temperature of the manifold. This means if a person comes in contact with it, they are less likely to be burned and leave skin behind. If there is a component close to it, it will not see as much heat as it would with an uncoated manifold. In addition, not as much heat will be radiated under the hood or into the engine compartment. This reduces the underhood temperature which, again, reduces the temperature of surrounding parts, such as, alternators and starters. It also reduces the amount of heat that can be drawn in through the carburetor, which is a secondary performance benefit."
http://www.cicceramic.com.au/coating...ifold-coatings
With the underhood temps of the XJ's whats people's thoughts on spending $300 on this... or just go with free mud
Cheers,