Adam Clarke on Psalms 51:9

Hide thy face from my sins - The sentiment here is nearly the same as that in Psa 51:3 : His sin was ever before his own face; and he knew that the eye of God was constantly upon him, and that his purity and justice must be highly incensed on the account. He therefore, with a just horror of his transgressions, begs God to turn away his face from them, and to blot them out, so that they may never more be seen. See the note on Psa 51:1 (note).