Scientific abilities rubrics developed at Rutgers University are an assessment and self-assessment tool. They consist of tables, each representing a broad scientific ability (for example, "to collect and analyze data"), the listing of subabilities (for example, "independent and dependent variables are identified") and the criteria to assess to which degree the subability has been developed. We introduced scientific abilities rubrics to assess student work in the "project laboratory," a project-based course where groups of students solve open-ended experimental physics problems. They submit a report in the form of a web page, which is evaluated and returned to the students with feedback on what needs to be improved. These iterations are repeated until the report is deemed acceptable. We present our experience with the rubrics, and the development of a new rubric for the web report. We show that the introduction of the rubrics reduced the workload of the graders, while increasing the quality of the reports. Based on these results we conclude that using scientific abilities rubrics is a very efficient way of assessing experimental project-based work. We also present an analysis of which abilities, assessed by the rubrics, appear to be the most difficult to develop.