I have a round honeywell thermostat model CT87A. Dust has collected on the mercury switch and spring. I've tried an air aerosol spray can but dust is adhered. What type of spray cleaner can I use?

NONE
Just put the cover back on and let it go. They all get a bit dusty.

If you start messing with the bulb it is a good possibility that you will move it.
If you move it it is a good possibility that the thermostat will go out of calibration and you can pay to have it recalibrated or buy a new one.