[Michal Janak]

I'm not super familiar with GIMP (Photoshop is pretty similar from what I remember, however). My guess is that you might be painting on the wrong layer? You will want to make sure that the Ambient Occlusion (AO) layer is above your paint layer, and the AO should also be set to Multiply instead of Normal. Hopefully this helps and isn't even more confusing! Throw up some screenshots of what you are looking at in GIMP and what you are having trouble with and I'll see if I can help you.

I found it, there was only one layer with parts of car body,  di made new group of layers and the orginal i set transparency to approx 80 percend and then it was OK ,

final result here