Note: Its a very good idea to name your layers. You can change the name of the background layer by double clicking it. I've named my layers 'metal tex' and 'metal colour'. Not very imaginative but I know at a glance what they are.
So now we add another new layer above metal colour. I've named this layer 'paint'. Fill this layer with 10,119,90. If you click on the little arrow on the far right of the colour tab & change to 'HSB sliders' if it isn't set already. Now we'll do a little painting. I used a large round brush (65) & painted some splotches on, using the HSB sliders to change the colours:
I then added a Gaussian blur, with a radius of 5 pixels.
Those splotches are looking too distinct. What I did to remedy that was to use the burn tool (65 brush again) on the light parts and the dodge tool (same again) on the dark parts. NOTE: I never use either of those tools on much more than 15 - 20% exposure as I feel its better to start subtle & build up, rather than have it higher only to find its too much. After that I used the blur tool (100 brush) on the whole thing & just smushed it all around until I was happy.
Ok, add a little noise again, somewhere between 1 & 2. I generally stick to Gaussian, but this time I unticked the monochromatic box. Play with it & see what you prefer.
Rule 3 of texturing: Chaos = real. Randomness including splotches, tonal shifts, noise, water streaks, dents etc all add believability to a texture. Go outside & look at where dirt collects on surfaces & where scratches & wear occur too. Take pictures if you can. You'll also get some sunshine & fresh air to boot!
Here's where it gets a little tough if you don't have the 3d total texture CD. Its time to add some grunge layers. The CD contains a number of grunge maps which are invaluable in creating dirty & worn textures. I'd recommend the texture CD, as it's good value for money & no I don't get commission. You can probably find some on the net for free, (I have)& its a good idea to build up a collection if you can. Failing that, you can create your own, either by taking pictures of dirty surfaces and then altering them in Photoshop by making them monochrome & playing with the levels, or you could paint them as well as using filter effects like noise or horizontal grain for instance, to enhance them. A word of warning - don't just use the clouds filter. It produces random fractals each time yes, but on the whole its easy to spot & doesn't produce great results. If you want to use it, my advice would be to use it in conjunction with other filters like noise or difference clouds, (try running that one about 8 to 10 times for different effects.) or hand painting as well. I generally only use it as a layer to alter the hue or saturation of a colour layer below by using blending modes. It's not just me saying this - Alex Lyndsey from DVgarage.com (an ex-ILM employee) says the same thing. Incidentally DV garage sells a grunge pack too - the surface toolkit, but its fairly expensive for the average artist. They do do some video tutorials that show how things wear & get dirty & what to look for when creating textures for free, which are worth checking out.
I added 2 grunge layers: on which was simply a picture of some concrete I found on the net, the other was a premade grunge texture which I used to add dark random spots to the surface.
Already it looks better. "But what was the point in having the first two layers Mike? We can't even see them!" Well hang on, we're getting to that soon. Ok, so for now, go ahead & add a layer mask to the paint layer, but don't do anything to it just yet.
Rule 4 of texturing: Your object/surface has a history. Think about what it is. What is the object used for? Is it used in the home or in warfare? Is it cleaned regularly? Does it bump against other stuff a lot? Think about how the object is used day to day. This will give you a better idea of where & how to place things like scratches, dirt & dents etc.
Ok, so now go to the channels tab & hit 'create new channel'. It should come up with alpha 1. We'll use this to add some dents. Fill the layer with white (255,255,255) & then make black the foreground colour. (Press 'd' for the default colours) I used a soft round brush (35) at 10% opacity to draw some dent shapes. Once I'd got the rough shape, I used dodge & burn to lighten & darken areas, along with the blur tool and smudge until I was happy. Don't forget that when doing this you're thinking in terms of height - with lighter areas being high & darker areas being lower. You'll see what I mean in a minute if you haven't already guessed. My advice with dents is not to get dent happy. Keep it subtle, depending on your specific needs.