I've been using IR thermometers for years. You just plain can't measure the temp of a mirror surface for and example. You will be measuring the temp of the item in the reflection. To measure a soda can for example you have to put on a piece of tape. Same for ductwork. A surface probe thermocouple would be the accurate way to measure but an internal probe into some thermally conductive goo of some type would be a decent makeshift. Only trial n error you can get an accurate e value for the different surfaces but once determined should work well. High temp tape should work well
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