Occasionally, poorly molded models will have sink marks and injector pin marks that need to be filled and sanded. The advantages to this approach are multiple. With the application of a little CA accelerator, the glue sets up instantly. CA is easy to sand if you tackle this task within a 24-hour period. The CA provides a firm, strong bond with the surface and is easily painted.
Many modelers are most comfortable using standard modeling putties.
These come in squeeze tubes like toothpaste and can be applied to the gap with an old hobby knife blade or a small paint spatula. If there are any imperfections, repeat the process until the surface is smooth. Most modeling putty has about the same chemical base as polystyrene cement, so they attack the plastic in order to adhere to it. It should be applied sparingly to avoid damaging the model. Finally, some modelers prefer using one of the many varieties of epoxies that are currently available.