For artists there is no definitive way to know when your painting is finished. That is good news and bad news. It is up to you, the artist, to determine when your painting is done. That gives you great freedom, but also responsibility for the success of the artwork. Some painters may work on a painting intermittently for as long as it remains in their studio under their gaze, not done until it leaves their possession; others produce so much work that they move quickly on to the next painting without looking back and reworking pieces; sometimes artists simply become bored with the artwork; and sometimes life gets in the way, leaving the work unfinished.
Painting is a process, and the same is true for finishing the painting. There is no specific endpoint. Rather, there are a series of possible endpoints depending on your goals and intentions. Here are some things to consider as you decide whether or not your painting is done.
The structure and bones of a painting can be achieved very quickly when you use a large brush and start with your larger shapes and masses. This underpainting stage of value and mass is often very beautiful, but many times artists continue beyond this point because they have a different objective in mind. While knowing what you want is good, it is also easy to lose sight of the goal near the end. It is not uncommon to labor over a painting, adding more and more detail, until the painting is seemingly lost.
Do you abandon your painting and stop when you feel that you have lost your original concept? Perhaps you could have stopped earlier, but since you didn't, now is the time to go back into the painting, painting over and eliminating some of the detail you already put in. Or you might consider setting this overworked painting aside and doing a new painting of the same subject. Having already worked out issues in the first painting, and with it fresh in your memory, you can now create a new painting more quickly with less labor and more vitality.
In painting, as in conversation, there are some things better left unsaid. Unless you are painting photorealistically, it is not necessary to include in your painting every detail you see. In fact, overly detailed work can be a distraction to the main idea of your painting and detract from its emotional power and impact. Too much detail can kill a painting.
Husband and wife artist pairs are often great critics of one another's work. So are artist friends. That's why working in a collaborative studio space is beneficial as is meeting regularly with artists for group critiques. Fostering friendships with other artists is crucial to growing and developing as an artist.
Give yourself some time away from your painting. Turn it against the wall for two days, or two weeks. Then look at it again. You will be looking at it with fresh eyes and will see it in a new way. You may suddenly see how to resolve a problem area and complete the painting. Or you may realize the painting is, in fact, finished as it is.
Make sure to always look at your painting from a distance. What you see up close changes dramatically when you step ten or fifteen feet away from it. Another way to do this is to take a photo of your painting and then look at it as a thumbnail. This is the way to see the masses, values, and Notan - the balance of light and dark - and to see whether you have maintained the integrity of your initial concept.
Look at your painting in a mirror. It is is amazing how this shift in perspective can help you to see your painting in new ways and to notice things that you might not have seen before. Also turn it upside down and on its side. See if it feels visually balanced to you.
Yes, this is an option, and many renown artists have deliberately chosen to do this! Unfinished: Thoughts Left Visible is an exhibit at the Metropolitan Museum of Art in New York City that runs through September 4, 2016. It includes works of Renaissance artists alongside modern and contemporary artists. It also includes paintings intentionally left unfinished - non finito - such as works by Titian, Rembrandt, Turner, and Cezanne, which engage and compel the viewer to fill in the gaps. It also includes works that were interrupted by life, as well as works that blur the boundary between constructed and de-constructed, such as those by Robert Rauschenberg. A beautiful catalogue of the exhibit, Unfinished: Thoughts Left Visible is available.
Perfection is a word that should be banned from art. There will always be something that "isn't quite right" to you as the artist. This is what propels us as artists to keep moving forward, learning, and creating. It is more than likely that what is bothering you as the artist is unseen to the average viewer. However, if your trusted critic also points it out, then it is well-worth addressing.
Determining when a painting is finished is an individual and subjective decision, just as is starting a painting. As long as you keep starting new paintings, chances are you won't get too bogged down in not knowing when to stop.