粉饰太平 To paint over and present peace. To hide disorder and show a picture of peace and prosperity.
捂盖子 To hold the lid down. To hide a problem and keep the truth from coming out.
遮丑 To cover the ugly. To hide a shortcoming, a mistake or something shameful with words or actions.
瞒上不瞒下 To hide things from those above but not from those below. Also, to act together so that superiors do not find out.