hitting the wall
I came across a well weathered wall while I was on holiday. Stood back about 20 feet and took a shot. This should represent looking at an O scale wall from about 4". Notice what isn't in the picture.
You guessed it; no nail holes. You can make out the odd nail head, and easily see indications of where the studs are. Lots of flaked paint down at the bottom, and lots of chalky (or dusty, the rain doesn't hit it up there) looking paint at the top. This illustrates the importance of modelling something by looking at real world references rather than based on impressions, memories, or word-of-mouth approaches. Find the actually item you want to model, or take a picture of it in good lighting conditions, and try to emulate what is really there.