I guess you have 2 ways to do it:
Procedurally, in that case it will depend on what render engine you use, but the Ambient Occlusion node, for example, works for both the 2 engines
With image texture, in that case you can use an image as a mask between 2 parts. You’ll need to unwrap your object.
A uv painted texture mask is probably the best method to go. Also you can use that same mask as the bump map as well so you dont get hard edges. External image painting software like gimp, sketchbook, photoshop would be just a couple to mention a few, the first two mentioned are free. Look up Andrew Prices gas container rust tutorial, he explains the process.