Normally, unless the condition of the cooling system is really bad, there shouldn't be any considerable build-up on the thermoswitch, not the eom or equivalent anyway. It might vary, but the only ones I've seen are yellow/gold in color, not silver :shrug: If one was made of a different material, that might increase the chance of build-up.
If you think that's the problem, I'd just replace it. The new one will come with a new o-ring, no need for any thread sealant. They aren't that expensive. Might wanna find out the proper torque to make sure the o-ring seals correctly, it was 17 ft-lbs. on my civic.