You can only cast Clear Casting Resin (which is a Polyester Resin) at a maximum of about 30mm at a time. This is because it does not have a filler content to it and thus overheats and cracks and crazes if cast thicker than 30mm. You can obviously build it up in layers, once one layer has cured. When building it up in layers you will not see any seam looking down through the cast, however you will see seams looking along the cast layers.
You should also note that you cannot cast Clear Casting Resin less than 5 mm thick! – It needs this thickness to generate enough heat to cure it properly.