What was David's hope for in his prayer for cleansing?
In "Biblical Perspective" Gordon Snider writes:
The need is greater than just blotting out sin. The psalmist longed for a totally new beginning. Scholars are amazed at the boldness of this petition. The word create suggests that David was looking for a miracle. In no other text is this word used to refer to moral changes in a person's life. This is the word used for God's speaking worlds into existence. This is a request for the removal of everything that separates man from God — a heart where no sin exists. Renew a right spirit within me. The term spirit occurs three times within three verses — twice referring to the human spirit, and once to the Holy Spirit. Here David, in parallel with asking for a clean heart, asked for an inner stamina to pursue right in every situation.
Source: Studies in the Psalms: Adult Teacher's Insights, page 48.