Boldini image from artrenewal.org
There are several things that have to happen before I call a painting finished. I take a picture of it with my digital camera and and either look at it on the computer screen or better still, print it out. I figured this out when I would see my work reproduced in magazines. I would often be surprised at how they looked reduced. Sometimes paintings that I though were fine had faults in them that I only noticed when I saw them shrunk down, Now that photography is digital and so instant it is any easy matter to check your paintings this way.
I generally let a painting sit around the studio for a week or so if I am not on a tight deadline. I also show it to my wife, or my kids, whoever, to get their opinion. Sometimes something will bother them about a painting that hadn't occurred to me. I ask them, does anything jump out or bother you?
There is an odd phenomenon I have noticed when doing this. Often they will be bothered by an area of the painting and say something is wrong with it. Often there IS something wrong in that area, but they are pointing at something else. They know the general area but not what the problem is.
After the painting has sat around in the studio for a week or so it should be dry enough to shoot with retouch varnish before you send it out to a gallery.You will also be able to look at it with a fresh eye after that much time has passed. Maybe something will jump out at you then. It is essential to fully troubleshoot a painting before it goes out into the galleries. I have gotten a few paintings back, and thought "what was I thinking". I had the painting out in the gallery for a year and all along it remained unsold, possibly because of one error that I had missed.
A GOOD PAINTING SHOULD CONTAIN NO ERRORS!
Did I mention that?
In art class they tried to be nurturing, and told you that there were no rules. Many art students don't want teachers telling them anything unfavorable about their art. But some pictures ARE better than others, and you want to be the one making better paintings. You may say, and I hear amateurs say frequently, "I don't want people judging my art". The only way you can avoid that is to put it in a closet. People will judge your art. When they do, the greatest compliment they can pay you is to buy it.