In engineering design, one of the most daunting problems in the design-through-analysis workflow is the dealing with trimmed NURBS, which often involve problematic topological information and lead to inevitable gaps and overlaps in the geometric model. Given the dominance of the trimming technology in the CAD community, fixing such a model with a watertight representation is highly desired. While remarkable progress has been made in recent years, especially with the advancement of isogeometric analysis, there still lack a fully automatic and scalable tool to achieve this reconstruction goal. To address this issue, we present a semi-automatic and scalable untrimming pipeline based on a scalable, field-aligned meshing tool QuadriFlow. On top of it, we add support for open surfaces to deal with engineering shell structures, and we perform sophisticated patch simplification to remove undesired slender patches. As a result, we obtain a watertight multi-patch NURBS with a nice semi-regular layout. Through several challenging models from industry applications, we will show that the manual intervention is minimal (i.e., tuning only a single parameter when necessary) and the pipeline can handle large-scale models within minutes.