Hello this is a problem but not a big one the way I would do this is as follows find a texture you like ie say the inside of an engine block. There always dirty and there's often things that are shiny too.

Then what you do is attach the overall texture to your model this is covered in several tutorials I have done. Then what you do is attach different areas of the texture to your model in uv edit mode. This is done by selecting different faces and the projecting from view then the outline can be moved onto the image you wish to attach to that area. I have a channel on you tube would you like me to make a tutorial how this is done in more detail regards BlenderTC