Psalms 51:2

Wash away all my iniquity

and cleanse me from my sin.

(v2, NIV)

In the first verse, David asked God to blot out his transgressions. What he has done is referred to as being recorded and he asks for that record to be erased somehow. Now in verse 2, he asks for cleansing from his sin and iniquity. What he has done is not just something to be removed from record, but something that has made him dirty. His uncleanliness from his sin is something he requests God for cleansing from. When we sin, we become unclean and dirty. We are no longer pure but are impure. What's also interesting is who does the cleaning. Sure, it may seem to make sense to ask for cleaning from God, but why doesn't he do it himself? Well, if you want to brush your teeth, do you use a dirty toothbrush? Or if you just took a shower, would you use a dirty towel to dry yourself? It something unclean is used to clean, and then what is being cleaned can't be properly cleaned. If something dirty is used to clean something, the thing being cleaned wouldn't necessarily get clean but it would probably be dirtied by the dirty thing being used to clean it. Similarly, David cannot cleanse himself because he himself is dirty. Instead, he asks God, who is holy and pure, to cleanse him from his sin. Only a cleansing from God would render you clean.