Why does Montag tell Clarisse that he could "never wash it off completely"?

In essence, Montag is referring to the fact that based upon what he does for a living, there is no way he can wash away the past or his own actions. In context, Montag's comment has a dual meaning. He is initially referring to the smell a fireman carries with him.... the kerosene.

"What-the smell of kerosene? My wife always complains," he laughed. "You never wash it off completely."

In another sense, Montag is talking about his undeniable part in the destruction of other people's lives.... the burning of their homes, the destruction of their lives, the times that life itself was taken. Over the course of the story, we see that the guilt of his actions is something Montag cannot wash away.