What is the hardest part of a kit to build? I take the question to be asking which part of the airplane is the hardest to build. I find wings and tail feathers easy, so that leaves the fuse. The fuse (to me) is the most interesting part; but, can be difficult to get built straight. When I get one done that I feel is exceptionally straight and well built, I feel I have really accomplished something. Building four wing panels for a biplane is tedious. It's easy and goes fairly fast, but, can get boring.