You can put any material within the MSO - which includes (AFAIK) a Composite Material which itself can have multiple sub-materials (images and masks) so you can put your rust on top of the main wing material as a “decal”.

From one point of view, yes - you have to create separate images for each part of the model, but individually they are easier to create than trying to do it all in one image where you’re constrained by the flattened UVW Map.

There’s not really a “right” or “wrong” way to do this - the choice is yours, but what IS important is understanding how UV Maps (in general) work and how to make best use of them. To do that you need to explore all the possibilities then use the most appropriate one (or the easiest) in each instance. Texturing is as important as the modeling if you want high quality results.

Max 4.2 through 2013.
XP-64 (SP2)
NVidia 9800GTX-512 (Driver 266.58).
Core 2 Quad Q6600 2.4GHz, 8Gb Ram, DX9.0c.