"Somewhat related to this, while using stryene plastic. You can take an xacto and make very thin shavings off a sheet of scrap stryene. Once you have a small pile you can add a few drops of Weld-on adhesive (for stryene) and mix it together to create a plastic paste that you can use to fill seams of your models. You can create seamless white stryene models with this technique, and it helps when it comes to sanding and painting to create an even smooth finish as opposed to spackle or even bondo spot putty. "