Methods, systems, and apparatus, including medium-encoded computer program products, for computer aided design of physical structures include: obtaining a first 3D model including a polygon mesh associated with one or more modelled solids, the one or more modelled solids being in a boundary representation format; producing from the polygon mesh a quad patch network that combines, using transfinite interpolation, the polygon mesh with one or more smooth boundary curves corresponding to the one or more modelled solids; defining one or more locally refinable smooth surface representations using the quad patch network as input and based at least in part on a smallest dimension representable by a geometry modeling kernel of a computer aided design program; and combining the one or more locally refinable smooth surface representations with the one or more modelled solids to form a second 3D model that is watertight at the one or more smooth boundary curves.
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