@Bancroft414 most of the time the plein air paintings need reworking because even if you captured the fleeting moment, after returning you will realize that there are mistakes or pictorially it might not be a successful one. So you make changes but the basic information is already there. You just start bumping paint by putting more light or shadow to turn it into a good one. Only work on accentuating things, light against shadow and shadow against light; correcting shapes. That's how it works.
For en plein air I believe that it is better that the artists go out and find out how they want to do. When you are out there the nature starts to paint. It is very different from studio practices. But you need to have an idea about how you can execute.