Thank you for the replies.
I thought I was a bit unclear as what I want to do. Maybe my intended work flow will help to better understand.
Create 3D model in Sketchup
Unwrap / Uv map model in Wings 3D. I know, I know, There are other better ones but I find Blender very intimidating.
Export the UV. map as a image to be "painted" on.
Import the Uv map image into sketchup and a apply to a square face.
Trace the face lines (shells) of each part using SU's line tool.
Apply the desired line style.
Export the face with all the lines to a new image but this image must be the same size as the original UV map image.
To be honest I am not even sure what line style was used in SU to create the grime / dirt layer.
I will try your suggestions and post again if needed.
Thanks once again.