Hi C17,
Are you working in Photoshop? If so, here are a couple of ideas.
First up, and this goes for all photo montage work, the placed object (SW in this case) needs to be just as messy and grotty as the background,otherwise it looks out of place. Start by running the Filter / Noise / Add Noise filter. Adjust the settings up and down until the SW resembles the background. This will give you some texture so the writing blends in better.
Secondly, applying text and graphics to a photo usually results in really hard edges. To reduce this, you can either run the blur tool around the edges of the graphic, set to around 10–20%, or you can Gaussian Blur the whole layer a little.
Thirdly, to get the background rivets and details showing through, try using different Blend settings on the SW layer, like Multiply or Darken or Overlay, and possibly knock back the transparency to make the layer slightly transparent.
I hope this helps!
Grubby.