A brand new client walked into my studio and declared, "My wife bought this little painting for $40 at a tag sale. It isn't worth anything!"
"Hmmm," I thought, "Even under the grime, I can see it's painted well..."
After a small test in the sky area, he agreed to let me clean it. As the days went by and the cleaning progressed, the painting began to remind me of the Hudson River School, and of a specific artist within that school - Jasper Friedrich Cropsey (1823 - 1900). Using strong magnification to search for a signature in the lower left, I found tiny, barely discernible initials in the dark brown foreground. After confirmation from the Newington-Cropsey Foundation (http://www.newingtoncropsey.com/jasper.htm) I found that the painting was indeed by Cropsey and could be valued near $40,000.
(After recovering from the shock, my client informed me that I am his wife's new best friend.)
To be clear, not every painting is a hidden gem, but cleaning and treatment by a qualified restorer often reveals beauty - and value.