The weathering is maybe the trickiest phase of modelling, because one risks to overdo the "worn out" look or to represent unrealistic effects; on the other hand, it is often the phase that allows to achieve a better realism.
First of all, I applied a coat of clear matt varnish, then I used a mix of brown and black paint in order to represent scratches and scrapings, which I painted with a fine paintbrush and with the "sponge" technique (that is, dip a sponge into the paint, then tap it onto the surfaces).
I worked mainly in the zones I believed more prone to wearing, like edges, handles and around the hatches. Moreover, in order to achieve a better depth effect, I painted a thin line along the edges of the scratches with a lighter hue of the surrounding camo color (this effect can be better seen in the first picture, along the edge of the lateral mudguard).
In the last phase of weathering I found confortable with pigments (those by Mig): they are versatile and can be used to get a broad range of effects. In particular, I used three colors for the rust shades:
First of all, I thinned some rust pigment with water (but you can use turpentine, if you want a stronger grip), applying it with a fine paintbrush over scratches, weldings and wherever rust can accumulate; after a few minutes I faded it with a clean damp paintbrush (with downward strokes for vertical surfaces, in order to create rust streaks).
I used five different pigments for the dust and mud effects:
In order to make dust marks, I applied some Europe Dust pigment with a soft paintbrush, blending it in some places with Light Dust and Concrete, thus varying the overall color. I applied more pigments in recesses and in the lower hull, that is where the dust is more likely to accumulate. The coat of matt varnish improved their grip.
This has been the trickiest part, but also the one I liked the most. First I mixed some Dark Mud with Europe Dust, soaking it with turpentine, then I applied it to the inner wall of the running gear, dabbing it in order to create a "muddy" texture; while it was still wet, I stippled it with small quantities of dry pigments, that is Europe Dust, Light Dust, Beach Sand and Concrete. The same operation has been done for the lower hull.
With the wheels it went slightly differently. Holding them horizontally, I put small piles of the same pigments I used before, onto which I dripped some drops of turpentine without directly touching them. The effect, once dried, is shown in the following picture.
For the tracks, I mixed vinyl glue with a bit of Dark Mud pigment and with some fine volcanic ash:
I then smeared this compound on the tracks with a small spatula (though removing it from the treads) and, once dried, I colored it with dry pigments. Track edges, teeth and treads, as well as sprocket teeth, have been highlighted with semi-wet silver pigment by Tamiya, in order to simulate the effect of metal polished by friction.
I coated the silencers with a mix of Track Brown pigment and turpentine, resulting in a nice base for rusty surfaces. Once dried, I colored them with dry Standard Rust and Ochre Rust, whereas I put some charcoal powder, also fixed with turpentine, on top of the exhausts.
On the front side, as an additional detail, I put a small chain, to which I gave a rusty appearance with the same technique I used for the silencers.
I finished the spare tracks with Ochre Rust and Standard Rust.
Finally, mainly to hide a main cannon I didn't like 😉 , I added a camouflage tarpaulin around the barrel end. I used a simple cotton gauze, tying it with a cotton thread, fixing it with vinyl glue and airbrushing it (Vallejo 70.889 Olive Drab). Finishing touch: some spots of Europe Dust and Vietnam Earth pigments.