You don't finish it. The viewer finishes it. When something is left unfinished, there is an energy that is still there, waiting. I often think of the great muscular slave sculptures of Michelangelo, struggling to be freed from the marble blocks imprisoning them. The mark on the paper are clues for the viewer who brings his experience to the process, fills in the gaps and get pleasure by participating.
Painting is spiritual.
A picture should not be Pablum for the eyes, making absolutely no demands on the viewer. I like to think of painting poetry. A prose passage, of course, is explicit in meaning, and every reader gets about the same message, whether in description, exposition or narrative. But with poetry, where words or phrases are dropped into fresh juxtapositions, each reader gets a slightly different meaning, and the reading, or viewing, becomes a creative process. According to B.P. Nichol, the definition of a bad poem is one that can be grasped all at once.