by GreatNortherner » Tue Jan 31, 2017 8:53 am
Hi,
My recommendation would be to use a near-black solid color as the base, and then overlay it with a semi-transparent photograph of some actual rusty metal. That usually comes out quite well and you can adjust the level of decay with the transparency of the rust layer, plus fiddle around with sharpening and/or color/contrast settings until you have reached the desired effect.
You can find generally free to use and awesome rust textures at textures.com, among other places. Also check out the "Decals" collection at Textures.com, there are some very useful grime/rust/smear/leak overlays in there, very handy with transparency/alpha already included
By the way, even on a clean, though not factory fresh, locomotive/wagon I'd recommend using some subtle dirt overlay on top of the solid texture to add a bit of "unevenness". Hardly any rail vehicle is 100% clean and shiny in the real world unless it has been washed or painted just moments ago.
Cheers
Michael