State standards could also help you with developing rubrics. Washington state where I live has developed standards for the arts. Going online to check out your state's standards or another state's standards might help you in framing not only a rubric for individual assignments but a rubric for your course goals. When you first presented the assignment what were you asking students to accomplish? What have you been emphasizing as the painting projects were progressing? When a student has an outstanding painting what is it about the work that makes it successful? Answers to these questions can help you in developing a rubric now even though you are already in the midst of working with these students. And it certainly can't hurt for the students to hear it now even if it is the end of the term. At least they'll know what their grade is based on. Also you might even get students to help develop rubrics. Perhaps you might say, "Consider what you've
learned this term in painting. What elements of art or techniques have been significant? What is an excellent example of that, a good example, a poor example?"