If you can paint a basic skin, which TBH if you cant your just wasting my time, it realy is extremely easy, if time consuming.
I try to find a photo of a rusty car, preferably the same chassis as the one you want to paint.
Simply use the ' dropper tool' ( I use PSP8 ) and select a rusty coloured pixel from the photo and simply start airbrusing away at the template, in the same general area as the rust is positioned on the photo.
Just keep on like that, selecting different various shades of rust colour as you go, and try to keep it as similar as the photo your copying.
The same goes for welding, but try and find a photo of some welded steel, its better if there is burnt paint and maybe some rust around the edges, and follow the same instructions as above.
Theres probably an easier way, but thats the only way that I know how to do it.
Its simple, but takes hours. Still, the longer it takes you to paint hardcore skins, the less time there is for you to waste looking at internet porn!
yea as crewie says thats ptretty much it though i only use paint brush,using diffrent blends of colours to get the raw look,that tina i posted up day b4 yesterday took me about an hour if that,quite easy once u get the hang of it,heres a couple i have done