Weathering means two things to me; battle damage and environmental effects. The former includes chipping, pitting and where paint has been damaged revealing the surface underneath. It's worth noting that depending on the impact or effect the surface underneath could also be other layers of paint. By combining varying degrees of damage you can create an easily interpreted appearance. Environmental effects can be physical like corrosion, mud, dust or debris or they can be atmospheric like scorching, heat discolouration or rain marks. There is no big secret to this, all these techniques are readily available online via YouTube and the fun is trying them out and seeing what effects can be created. I kept it simple with the Caestus and focused on just a couple of atmospheric effects over the battle damage. Firstly a light coat of thinned Tamiya flat earth applied with the airbrush to simulate dust and grit and finally some atmospheric and melta scorching using Tamiya flat black, again applied with an airbrush. The final task was to provide a suitable flying base for the XIII legions latest transport.