ideally just screen shots of different graphs showing the various techniques.
for me, the thing I have trouble with is paint on metal, I get really confused. the metal is a metallic substance, but the paint itself isnt metalic, its a plastic, so if anyone could give some advice on that one it would be a big help.
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Many of these materials can be opened in Substance Designer so you can step through the graph and see how people have set up their materials.
Edit: As for your question about painted metal, you're basically going to need to create 2 separate sections in your graph, one section will be your base/metal, the other section will be your paint. You'll then mask between these for all of your outputs (Metallic/roughness/albedo etc). Anywhere you have chips or scratches on the paint and you want the metal to show through you will mask the paint portion to reveal the metal.
If you have the subscription, you can also look at the official Allegorithmic materials on Source. Not everything has the .sbs format though. Same goes for Share actually, so some things you download might not have a graph for you to look at.