Lately I've working on trying to improve on my understanding of materials. There are quite a few amazing resources out the showing different processes. Arthur Gimaldinov has a great tutorial on painting wood and Alex Grishin has a nice assortment of videos which you can find on his Youtube channel.
They're a great start if your just starting out and looking for place to begin. But the best way to study is from reference. To that end, here's a piece of wood.