Once the first base layer of gesso (or air dry clay) is dried, you can sand the flutes on the first layer of gesso using sandpaper rolled over a rounded toothpick or file handle and begin to build the fluted face of the sink up with a second layer of gesso while you add an apron edge.
To add the apron edge, a straight edge scrap of wood or plastic is laid up against the front upper edge of the sink, and extra gesso is placed over the upper edge down to the form. Once the gesso is partially dry, the form is removed gently. When the gesso is fully dry the edge can be sanded with sandpaper over a sanding block, and any holes filled with more gesso.
Continue building up the flutes and sanding them, and building up and sanding the apron edge. If you want both flutes and an apron edge, it is probably easier to get the flutes fully built up first, and sanded to their final shape, then add the apron edge. In my sample I tried to build both the flutes and the upper edge at the same time, which is a problem when you sand the flutes.