Surfaces in the temperature range of 140F will burn bare skin, setting the upper safe level as referenced within the ASTM C1057 Personnel Protection standard. This standard is based on the principal of being able to safely touch a hot surface with bare skin for 3-5 seconds. The standard was established using a thermesthesiometer, a device that measures heat transfer in a way that's similar to human skin.
Surface temperatures up to the 350-400F range can be reduced to safe levels with ceramic insulation coatings. The 320F-350F range may be considered a practical upper limit to ceramic coatings if there's a high probability of exceeding that range for extended periods of time. If that upper limit is not exceeded then protection can be provided with thicknesses as low as 0.080-0.120". Ceramic coating can be applied while equipment is in service with the benefit that application time can be significantly reduced.
PERSONNEL PROTECTION IS BASED ON A SURFACE TEMPERATURE OF ABOUT 138F/
The following thicknesses provide general guideline for protection in the stated temperature ranges. Keep in mind they're subject to ambient temperature in that the highter the ambient, the net effect is a higher surface temperature. For more details regarding Personnel Protection (PP) based on 60C/137F per definition by ASTM C1055 and ASTM C1057 together with ISO 13732-1:2006, just Google it.
For up to 200F - 40 mils/1 mm
For up to 250F - 60 mils/1.5 mm
For up to 300F - 80 mils/2 mm
For up to 350F - 100 mils/2.5 mm