Capacity to teach the arts is a problem reported by many teachers in primary (elementary) school settings in Australia. This paper reports on research which explored how to build primary school teachers' capacity in arts-based pedagogy. It outlines the design and development of a co-mentoring program between arts mentor practitioners and generalist primary school teachers which used an Arts Immersion approach. The findings of this research reveal the effectiveness of co-mentoring as an approach to support professional learning in arts education, and the use of an Arts Immersion approach to improve teachers' capacity in planning, facilitating, and assessing authentic arts experiences.