Unfortunately, you will have to sand down the set epoxy in order for the new layer to bond. Even then, while it will stick to the other epoxy, it will not be as strong and will not give you a long-lasting end result. If you wish to do this, you will need to take a sander and run it over the surface with 180-grit sandpaper. It is important that you then clean off the surface. You will now be able to allow a chemical bond to form between the two epoxy surfaces, and will therefore have a seamless epoxy project.