I modeled an old car and rendered it out. Now it's like just manufactured and no marks of usage. What are good ways to add dirt and wear to my car? There's a lot of parts with solid colour so I didnt make uvs for those, do I need to uv everything out at first and then paint the dirt and scratches in photoshop?
What do you think about the model? I'd like to have some feedback. I try to achieve realistic model but as you can see, it's still quite cg looking. I bet dirt and scratches add a bit realism to it. What else should I change?
It depends on where you want the scratches and how 'worn' the car is supposed to be. Maybe just the fenders will do the trick. And those should be too hard to UV.
Regarding the model, it does look fairly CG-y. One thing that helps sell your model is minimizing the number of perfect corners. The back of the car has perfect corners. In the real world, it's really hard to find a perfect corner. They're almost always rounded of to a certain degree. So give them a bevel when you're nearly done. Don't do it immediately, because editing beveled geometry isn't as easy as non-beveled.
Besides that I don't think the problem is really in the model. I think it's the lighting and the shaders. Put an HDR image in there, look into some more advanced shading tutorials.
I agree with Ben's comments. Also, it looks like you've used a procedural texture, maybe a fractal, for the bump map on the roof. Fractals will always look fake. Create a more realistic bump map using photo references and hand-painting in Photoshop.
Also, one general tip for grunge texturing: the key is always to have your various texture maps correspond well with one another. A scratch or scuff needs to change the colour, the spec, and the bump all together to produce a convincing effect. Generally you'll want to create one of your maps, say the colour map, and then generate the spec and bump (and maybe diffuse, reflectivity, etc.) maps from that.