I think this will be the last entry on this subject, having started back in February. It all got done within a year, which is good going for me. Here's the picture of the finished model.

In the end the salt weathering didn't work out very well. A lot of the salt crystals come off during airbrushing, so for any decent rust pattern you need to start with lots and lots of salt. Much more than you'd think. Because of that it's difficult to create the pattern you want. It's kind of random. Plus there is a lot of salt to clean off afterwards, it gets everywhere. In the end I went back to the paintbrush to create the base rust pattern and then added some airbrush over the top. I'll practice with the salt technique because I can see it has potential for some effects. There are also other techniques from the military modelling fraternity that might be worth a try, such as chipping.

Having studied a lot of steel mineral wagon photos one thing that is apparent is that the rust effects are varied, and typically not just a few blobs and streaks of a uniform red brown colour. A fair proportion of them have the kind of pattern I've tried to represent here. There are patches of old rust that are a very dark brown, these are well defined and can be applied by paint brush. Around them there is often a kind of black/brown haze which really needs an airbrush to apply. I'm not sure what the haze is, corrosion spreading to surrounding paint, coal dust adhering to to the area. Don't know but it is a distinct and common weathering effect. And very little red/brown rust. The colour of rust is dependent on the chemistry of oxidising iron and that changes with time and environmental exposure. It often starts as an almost yellow deposit, darkening through orange/browns, darker browns and in some cases ending up as a deep purple/brown. Some of the lighter oxides stain the surrounding paint, often getting washed down in streaks by the rain.

One other thing I've noticed in 7mm is that texture starts to become important. As it happened the legacy of the salt weathering left some texture (aka mess) that fortunately added to the effect. Since I paint mostly in acrylics there days there is the option to add some acrylic resin putty to areas of paint to create some subtle texture.

Almost finished - the brake pin chain and one of the top flap pin chains need to be replaced.