The face you put on when shaking your bosses hand or meeting you parents old friends. Its a smile and nod, but you have no idea why you smiling. also used when someones telling a very bad joke, but you want to make them feel better.
-"oh look at Matt shaking Mr.Hayes hand, he has a total plaster face on"
-"ha and now hes laughing at mr.Hayes terrible jokes, but its a total plaster face"