I suppose I would consider any plane you build to be a practice plane, unless it's the last plane you plan on building. Building is a bit of an art form. Each plane should get better with experience. That TF P-51 isn't really a diamond, but it's no dog either. Do your best work on it and be proud of your accomplishment. Just know that your next plane will be better from the P-51 experience.
Painting boxes before the plane? Probably good advise. Once you settle on a paint and technique you feel comfortable with, you will only get better.