Engine Exhaust Heating of Cabin Windows and Airframe
1. Do not operate the engines with the propellers feathered except during external power starts and propeller feather checks, except that the propellers may be operated in feather at temperatures not to exceed +5*C for a maximum of 3 minutes for purposes of airframe deicing.
That is straight out of my company ops manual. From personal experience, to be used as a guideline along with a bit of common sense.
Cheers
DB
