Sarah Clarity Studios

Chapter 2: Mud, Dust, Oil & Streak Logic

Created by Sarah Choi (prompt writer using ChatGPT)

Mud, Dust, Oil & Streak Logic for Mecha: Wear, Weathering & Damage States

Grime is physics you can see. Mud, dust, oil, and streaks are not random overlays—they are the visible result of motion, airflow, gravity, surface finish, heat, and maintenance habits. When grime is placed with logic, it does three high-value jobs at once: it sells scale (micro vs macro deposits), it sells environment (desert, swamp, shipyard, industrial city), and it clarifies form (dirt in cavities, clean high-contact edges). For concept artists, grime logic is a fast way to make a mecha feel real without drowning it in micro detail. For production artists, grime logic is a layering plan: masks, material responses, and state-driven variations that stay coherent across LODs and lighting.

A useful mantra is: dirt has sources, paths, and sinks. Sources are where grime is generated or thrown. Paths are how it moves (flow, splatter, drip, smear). Sinks are where it settles (cavities, seam bottoms, behind brackets). If you can explain those three, your weathering will look intentional.

The four laws of believable grime

First, grime follows gravity. Drips run downward, pooling at seam bottoms and edges before dropping off. Second, grime follows flow. Airflow and motion create streaks and clean zones, especially on leading edges. Third, grime follows texture. Rough surfaces trap dirt; smooth glossy surfaces shed it or show it as smears rather than buildup. Fourth, grime follows touch. High-contact areas get rubbed cleaner but also get oily smudges.

Concept artists can think of these as a composition tool: you’re placing darks where the form needs grounding and leaving lights where the silhouette needs clarity. Production artists can treat these laws as layer rules: gravity streak masks, flow direction masks, roughness-driven dirt adhesion, and touch-driven polish/smudge.

Mud vs dust vs oil: different materials, different behavior

Mud is a wet particulate paste. It splashes, cakes, cracks as it dries, and clings in thick deposits—especially in undercarriages, foot cavities, and behind protrusions. Dust is dry particulate. It settles as a veil, collects in cavities, and gets blown off leading edges. Oil is fluid (or greasy mist). It leaks from seals and fittings, spreads as a sheen, attracts dust, and creates glossy streaks.

A common mistake is using the same brush language for all three. Mud has volume and edge thickness. Dust has soft gradients and occlusion buildup. Oil has a wet specular signature and tends to form streaks and halos around fittings.

Start with the environment: what’s in the air and on the ground

Weathering is environmental storytelling. A desert mecha gets fine dust that forms soft veils, with sandblasted leading edges and dust packed into grilles. A swamp mech gets wet mud that cakes, with splash patterns and dried crack networks. A snowy mech gets slush and salt-like streaks, with water runoff and freeze-thaw staining. An industrial city mech gets soot, ash, metallic dust, and oily grime.

For concepting, decide the dominant particulate and moisture level. Then pick two secondary contaminants (soot, oil mist, salt spray). That combination will produce a distinctive grime signature. For production, these choices become a set of environment presets: different dirt albedos, roughness behaviors, and mask intensities for each biome.

Sources: where grime is generated or thrown

On a mecha, major grime sources include feet and lower legs (ground contact), exhausts and vents (soot and heat-baked deposits), hydraulic and coolant systems (mist and leaks), weapon use (muzzle soot, blast residue), and maintenance (grease, fingerprints, solvent streaks). Tracks of motion matter: a biped throws mud forward and outward on the shin and knee guards; a quadruped may splash under the belly and inner limbs.

Concept artists can mark sources with small callouts: “mud throw,” “soot plume,” “leak here.” Production artists can create source masks tied to specific components—thrusters, vents, joints—so the grime layers remain consistent across skins and damage states.

Paths: splatter, drip, smear, and streak

Splatter is ballistic. Mud and water splatter in arcs that reflect leg swing and speed. Splatter patterns are directional: they hit the front of shins, the underside of thigh plates, and the rear of calf guards depending on gait and acceleration. Drips are gravity-driven: they start at a seam, fastener, or edge, then streak down and break into drops.

Smears come from contact: brushing against walls, kneeling, sliding, or hands wiping grime off. Smears often create cleaner streaks inside a dirt field, which is a great readability cue. Streaks are flow-driven: rain streaking, airflow streaking, and heat plume streaking. Each streak has a start point (source) and a taper (flow/evaporation).

For concepting, place a few strong, readable streaks rather than many faint ones. For production, build streak layers that respect surface curvature and have believable breakup, with separate behavior for wet drips versus dry dust streaks.

Sinks: where grime settles and stays

Sinks are where dirt “lives.” Cavities, seam bottoms, inside corners, behind protrusions, under overhangs, and around gaskets are prime sinks. The underside of armor plates, the back side of vents, and the bases of bolts often show grime halos. Sinks are also where corrosion begins because moisture stays longer.

Concept artists can use sinks to enhance form: darken cavities and seam bottoms, especially on the lower half of the mech, to ground it. Production artists can use ambient occlusion and cavity masks as a base, but should add authored sink masks for key areas like knee pits, ankle housings, and undercarriage recesses.

Finish matters: matte traps, gloss sheds (but smears)

Surface finish controls adhesion. Rough matte paint holds dust and looks uniformly dirty quickly. Satin surfaces hold less and show directional streaking better. Gloss surfaces shed dust but show fingerprints, smudges, and water streaks as distinct patterns. Rubber gaskets trap grime at edges. Ceramic thermal coatings often hold soot as a powdery layer and can show baked-on stains.

Concept artists can depict this by varying edge softness: dust on matte is soft and even; smears on gloss are sharper and more contrasty in spec, not necessarily in color. Production artists should drive most of this through roughness changes. A “dirty” surface is often less about darker albedo and more about altered roughness—either rougher from dust or glossier from oil.

Oil logic: halos, streaks, and dust magnets

Oil shows up where something moves or seals: actuator rods, rotary couplings, hydraulic fittings, and access panels. A small leak creates a halo around a fitting, then a gravity streak. Oil also creeps along edges and collects at seam bottoms. Over time, oil attracts dust, creating a darker, stickier grime that looks different from dry dust.

For concepting, oil is a great way to make joints feel functional: a subtle dark halo at a piston seal, a glossy streak down an ankle housing, a dust-caked edge where oil has trapped grit. For production, oil layers should modify roughness strongly (wet sheen) while keeping albedo changes modest. Add a secondary “dust-on-oil” layer to sell age.

Mud logic: thickness, cracking, and shedding

Mud buildup is about thickness and breakup. Fresh mud is dark, wet, and glossy in spots. As it dries, it lightens, becomes matte, and cracks—especially on thick deposits. Mud sheds from high-impact zones and gets knocked off edges, leaving “clean bite marks” where it broke away.

Concept artists can show mud thickness with edge lips, clumps in corners, and a few cracks. Keep mud heaviest in lower regions and undersides. Production artists can implement mud as a height/normal layer with edge thickness cues, but should be careful about LOD: thick mud shapes must read in silhouette or broad normals; tiny crack networks can be close-up only.

Dust logic: veils, gradients, and clean zones

Dust is about gradients. It settles from above, so upper horizontal surfaces can get a veil. It also gets pulled into cavities and grilles. Movement creates clean zones: leading edges and high-flow surfaces get wiped, creating “wind-swept” patterns. Dust also gathers in patterns around static electricity and heat plumes, but the most believable cues are simple: top dust, cavity dust, and flow-clean edges.

For concepting, dust is a readability tool: soften and unify large areas while keeping sharp edges and key features cleaner. For production, dust layers should primarily increase roughness and slightly lift albedo (dust lightens many surfaces), with cavity accumulation controlled by masks.

Streak logic: rain, coolant, soot, and salt

Streaks come from repeated wetting and drying. Rain streaks start at upper edges and drip points, running down and breaking where the surface changes angle. Coolant streaks can leave colored residues and “tide marks” where fluid evaporated. Soot streaks follow exhaust flow and often taper with distance. Salt streaks (marine environments) leave chalky deposits and accelerate corrosion at seams.

Concept artists can create a few “hero streaks” that imply the rest. Production artists should treat streaks as layered decals or masks with directionality and breakup. Avoid perfectly straight streaks; real streaks wander, split, and follow micro channels.

Integrating scuffs, chips, heat, and corrosion with grime

Grime is most convincing when it interacts with wear. Scuffs create cleaner patches inside dirty areas. Chips expose primer and metal, which can rust and also collect dirt at their edges. Heat zones bake grime into a different roughness signature—soot becomes more permanent and slightly glossy or chalky depending on material. Corrosion often appears where grime and moisture linger: seam bottoms, behind brackets, around fasteners.

For concepting, link these effects in small chains: “chip → rust bloom → streak down,” “heat zone → soot deposit → baked halo,” “handle → clean rub + oily smudge.” For production, build these chains as layered materials where one mask feeds another: chip mask drives rust; oil mask drives dust adhesion; heat mask modifies soot roughness.

Damage states: how grime evolves over time

A clean mech has crisp materials and minimal deposits, but it still has touch smudges and subtle cavity dust. A used mech has patterned grime: dust veils, mud splashes, oil halos, and streaks that reflect routine operations. A damaged mech can have chaotic grime: leaks, exposed substrate, heavier soot from compromised exhaust, and corrosion accelerated by missing coatings.

Concept artists can plan these as A/B/C variants and keep the grime logic consistent—same sources and sinks, just intensified. Production artists can implement this with scalable parameters: dirt intensity, wetness, leak severity, and soot amount, driven by gameplay or customization.

Production translation: a grime layer stack that stays coherent

A reliable grime stack often includes: a cavity dirt layer (sinks), a dust veil layer (settling), a streak layer (gravity and rain), an oil/leak layer (fluid sources), a mud/splash layer (ground contact), and a soot/heat layer (exhaust and weapons). Each layer should have its own mask logic and material response—especially in roughness.

Concept artists can support production by delivering a “grime map” paintover that marks sources, paths, and sinks. Production artists can then build reusable smart masks: lower-body gradient for mud, world-up dust accumulation, world-forward travel wear, and component-tagged leak sources. The result is weathering that looks authored, not procedural.

Closing: grime is readable history

Mud, dust, oil, and streaks are the readable history of how a mecha moves through its world. When you treat grime as sources, paths, and sinks—and you respect gravity, flow, finish, and touch—you get weathering that feels inevitable. For concept artists, grime logic gives you quick, high-impact realism and better form clarity. For production artists, it provides a repeatable layering system that holds up across LODs and damage states. The mecha doesn’t just look dirty; it looks lived-in.