I like to use Liquid Green Stuff to fill in gaps on my models. This is especially great for small seems like on arms, legs, etc. I’ve learned quite a bit on how to create nice, smooth, seamless blends with it, so you don’t have to do any sanding. It takes a bit of practice to do, but once you master it you can fill gaps quickly and easily.
View the Tutorial: Liquid Green Stuff and How to Use It Easily for Gaps