Here is the technique that I use to get most of my textures completed. It has been refined over a period of 8 years. Enjoy!
Usually I make sure I have the model completely finished before I start anything.
Then I UV unwrap the object. For this I did NOT put similar UV islands over each other to save space, instead everything has its own pixels for maximum variation.
After that I bake the ambient occlusion so I can get a good idea where to put the dirt/scratches. This is way more helpful then saving the UV coordinates as an image.
Then I start on the super fun part, texturing!
For texturing, this is really the ONLY spot you can give an object a story. When coming upwith the texture keep these things in mind:
Where has it been?
How was it used?
How well was it taken care of?
How long has it been used?
For the wheels:
Where has it been?
Through a bit of mud,rocks, now traveling over super soft dust.
How was it used?
To pull a heavy vehicle over hard objects at race pace.
How well was it taken care of?
It’s been beat up but constantly serviced (this explains the dark dirt/grease and scratches on the bolts)
How long has it been used?
For a while, this is a low budget off road rig
Of course while you are texturing you can keep adding to this list.
Also get a ton of references. The more you get, the more realistic it will be .
When I am doing the scratches, I have two different work flows I use on the same texture.
I made a Photoshop scratch brush that is pretty thick with a bit of scattering and a lot of size variation. (I will attach a picture of the settings later, I am on a different computer at the moment). With that, I trace around all the edges that would get scratched constantly. It will look really even at this point.
I temporarily removed the ambient occlusion layer for all the images so you can see the texturing details better.
So make a mask, then increase the scratch brush size and variation. Paint over it like a 5year old who just had 10 candy bars. This will remove the solid scratch lines. This is where I control the scratch amount. The less Iwant, the more I paint over it on the mask. Also be sure to remove any scratches where it logically would not be.
The Result:
The mask:
This will give you the big scuffs, but we still need the little long scratches.
For this, have less variation, decrease the bush size and make a new layer. Then draw on that like a hyper 5 year old again (Now you see why I like doing this stuff).
Once again, add size and variation then pain on the mask to make the scratches look more random.
Both scratch variations together:
Now for the dust!
For that, think of where the dirt would stick, keep in mind when the vehicle is in motion, the wind resistance will blow off a lot of the dust. Because of this, one should gather many references of similar dusty objects.
You can get away with the same scratch brush that was used before. Just lesson the variation by 3a lot, turn the opacity down so everything looks softer and more like dust.
Of course mask off the areas where dirt should not be, then start painting.
For the grease:
The grease will be near mechanical devices or surfaces. So around the edge of the hub cap I added grease (which turns into dark dust). I then added some dark dust around the bead lock bolts. (In this case it is anti-seize, but will still end up as darker dust).
The Scratches, dust, and grease are separated into their own black and white masks for cycles.There I can tune the material for each. Example: Change the color of the dust, make it more opaque or translucent, change the glossiness etc…
