UV question

I’m just starting to learn UV mapping and gimp so i will assume my problem is simple to fix.

I’ve modelled a car and have figured out how to unwrap it okay. I’ve started to practice the painting of the uv map in Gimp. I am just trying to darken some edges and simulate a little road grime by painting over a transparent layer. everything seems to work fine with the painting part but when I insert the image on the UV map and render it the transparent layer seems to cancel out the existing blender material color of the mesh.

Any ideas?

Thanks

Im not sure whats happening so I will just go through the whole process.
So you already have some materials and/or textures for the car and you want to add an image texture on top of the existing ones. So after lining the grime texture up in the UV window, follow these steps.

  1. Add a new image texture to the car, selecting the grime image.
  2. Click the use alpha button if the image format supports alpha(tga) or click calc alpha if the image texture has a black background.(If all else fails hell click em both)
  3. In the Map Input tab click the UV button to use your UV positioning.
  4. In the Map Output tab, click the color AND alpha buttons.
    Now it should work (I hope). You might also want to see what it looks like with the Nor channel button pressed also.
    If you did all this already and it still isnt working, try adjusting the Var and/or DVar sliders in the Map To tab.When using the Calculate Alpha method, it looks like setting DVar to zero and Var to 1 is what you want.