As world-class modeler Shep Paine observed in the
Armor Modelers Guide: “Tanks love dirt. They wallow in it. They don’t move over the terrain so much as through it, joyfully spreading mud, dust, and dirt all over themselves and the surrounding countryside.”
So, it’s important to be able to model those conditions on scale armor. Dust and light dirt are easy to make with pastels or powders.
Mud, on the other hand, is thicker and tends to have lumps and chunks, so you need something that adds texture and realistically clings to the model. Some modelers use materials like Celluclay or modeling putty, often applied before painting.
I prefer to apply mud after painting because that’s how it happens in the real world. But be careful, because applying any kind weathering to a model may cause a reaction with the paint underneath are ruin your carefully rendered camouflage.
My secret for this method is acrylic gel medium, available in the at art supply stores or online. Gel medium comes in consistencies from thin to thick and in finishes from flat to glossy. Choose a viscosity and finish that suits the type of mud you want to model. Flat is good for old, dry mud; gloss for fresh, wet stuff just churned up.