Partially, I don't throw them out because I always think that I might be able to use one bit of something as a building aid. For example, I stripped and covered a wing to practice my silk over Koverall technique and rib stitching. And you can see where I've been practicing hand-painting German crosses on the Cub wing. By the way, here's how the final wing came out thanks to lots and lots of practice on these discarded bits.