I remember awhile ago when a student kept having faint, white fingerprints on her prints. I told him to wash her hands because she may have had some lotion or other oil on them. But no matter how many times she did this, she still got fingerprints.
Then I saw her using her dodging tool with plasticene on it. (Her father told her about that handy hint.) She was reshaping the tool or just touching it every time and that was depositing oil on them.
Once I figured it out, the fingerprints disappeared.