(one) does not wash (one's) dirty linen in public
One does not discuss very private, personal matters, especially those that may be embarrassing, in public or with other people. It always makes me uncomfortable when John starts going into all his personal problems whenever our friends get together. I do not wash my dirty linen in public, and I wish he wouldn't either! Kids, please don't say anything inappropriate in front of your grandmother—she definitely believes that one does not wash one's dirty linen in public.