There are a few ways to fill the gaps in plastic model kits. One way is to use a liquid model cement. This type of cement is thin and will flow into the gaps between the pieces of the kit. Another way to fill the gaps is to use a putty-like material. This type of material can be molded into desired shapes and then used to fill in the gaps. Finally, you can also use scraps of plastic from other parts of the kit to fill in the gaps.