Kits, especially Armor, need mud and grime to look realistic. Tanks rarely stay brand-spanking-new for long. During wartime aircraft don't have time to go to the paint barn or wash rack and will accumulate dirt and chipped paint quickly.Dirt: Armor will get covered in a layer of fine dirt whether in a forest, jungle or desert. Aircraft accumulate black, sooty, oily dirt on surfaces and in crevasses from the air stream during flight.
Dirt created with a paint Wash on the floor and Drybrushing any raised surfaces. Dirt Drybrushed on the inside and vehicle tops, and splattered on the running gear with a Wash. Mud: Armor will get covered in dirt. Tracks, treads, road wheels, and undercarriage will accumulate thick mud, grass, tree branches, and leaves. Mud created with putty and fine sand. Mud created by mixing white (Elmer's) glue and fine sand, and then dabbed on with a toothpick. Using real sand produces a natural dried mud color, so there is no paint required. Leave any small twigs, grass, or leaves to add realism. When the mixture is dry, dark brown can be painted over the thicker inside portions of the sand to resemble wet, fresh mud. While leaving the thin outside edges the natural sand color to resemble drying mud. Engine and Gun Exhaust: Both of these create black soot. Exhaust pipes, and anything near them, and gun barrels and brass ejection ports, will be surrounded by black grime. Best rendered with an airbrush. Goto: Airbrushing for more details. 
Damage: Tanks' fenders get dented, armor plates become bent, and tools and accessories break off. Aircraft acquire bullet holes and battle damage, and normal wear and tear. 
Thin and Bend fenders and grills. Melting the plastic works, but be careful and have an adult present if you are not one. Windshields and Canopies:Paint an extra thin wash over the clear plastic, masking with tape to create the wiper blade path. If some paint seeps under the mask don't worry, it will look more realistic since rain still runs down the glass after the blade has passed! Paint Chips and Wear: Aircraft leading edges and maintenance panel areas lose paint and show bare metal. On armor the paint will get scratched and the steel will rust. Leading edge wear can be done with aluminum paint and a small brush, or a Q-Tip barely wet with paint and rubbed over the edge.
Rust: Mainly for Armor on Tracks, Bare Metal and Exhaust Stacks.
