Weathering refers to making a model look used and exposed to weather by simulating dirt and wear on real vehicles, structures and equipment. Most models come out of the box looking new, because Non-weathered finishes are easier to produce and many collectors want models to look pristine. Also, the wear a freight car or building undergoes depends not only on age but where it is used. Rail cars in cities accumulate grime from building and automobile exhaust, while cars in deserts may be subjected to sandstorms which etch or strip paint. A model that is weathered would not fit as many layouts as a pristine model which can be weathered by its purchaser.
Weathering purchased models is common. At the least, weathering aims to reduce the plastic-like finish of scale models. The simulation of grime, rust, dirt, and wear add realism. Some modelers simulate fuel stains on tanks, or corrosion on battery boxes. In some cases, evidence of accidents or repairs may be added, such as dents or freshly-painted replacement parts, and weathered models can be nearly indistinguishable from their prototypes when photographed appropriately.