Writing centers are learning settings and communities at the intersection of multiple disciplines and boundaries, which afford opportunities for rich learning experiences. However, navigating and negotiating boundaries as part of the learning is not easy or neutral work. Helping tutors shift from fixing to facilitating language and scaffolding literacy learning requires training. This is particularly true as tutors work with second or subsequent language (L2) writers, a well-documented area of tension. This mixed methods action research study, conducted at a large university in the United States (US), centered on a tutor training intervention designed to improve writing tutors’ scaffolding with L2 learners by increasing tutors’ concrete understanding of scaffolding and shifting the ways tutors view and value L2 writers and their writing. Using a sociocultural framework, including understanding writing centers as communities of practices and sites for experiential learning, the effectiveness of the intervention was examined through pre- and post-intervention surveys and interviews with tutors, post-intervention focus groups with L2 writers, and post-intervention observations of tutorials with L2 writers. Results indicated a shift in tutors’ use of scaffolding, reflecting increased understanding of scaffolding techniques and scaffolding as participatory and multidirectional. Results also showed that post-intervention, tutors increasingly saw themselves as learners and experienced a decrease in confidence scaffolding with L2 writers. Findings also demonstrated ways in which time, common ground, and participation mediate scaffolding within tutorials. These findings provide implications for tutor education, programmatic policy, and writing center administration and scholarship, including areas for further interdisciplinary action research.