I just got a temp gun, more like a dongle. It gives different results depending on the surface of the object. Shinny motor cans read much lower that they actually are. What do you do about that?
The energy received by your I/R thermometer is determined by the "emissivity" of the surface you are measuring. The more reflective the surface at the wavelength you are measuring, the lower the emissivity. Unless there is an emissivity correction setting, the lower the emissivity, the lower the indicated temperature. Bare metal can be rather low.
The good news is that the inexpensive I/R thermometers operate at very long wavelengths (around 8 microns) and at those wavelengths, most paints and organic surfaces are equally emissive at around 0.9 as I recall. So your solution of painting the motor can is a good one, since that is probably the emissivity that the thermometer is set for.
The other factor is the spot size of your measuring device. If it is larger than the surface you are measuring, you will get a low reading. Unless your device has some serious optics, the spot size is probably larger than you think. If it has a laser aiming device, that does NOT indicate the actual spot size.
Sorry, long answer. I used to be a sales rep for an I/R temperature measurement company. Kinda got the juices going again.....