cover up

blow someone's cover

To reveal that someone's behavior, situation, or identity has been fabricated or deliberately misrepresented for an ulterior motive.

break cover

To disclose one's real thoughts and intentions.

cover one's bases

To be thorough; to prepare thoroughly or completely.

cover someone's ass

To make preparations or take precautions to ensure that a person is not blamed or punished for his or her conduct.

judge a book by its cover

To make judgments about things or people by considering external appearances alone.

take cover

To shelter oneself.

you can't judge a book by its cover

It is not possible to make reliable judgments about things or people by considering external appearances alone.