The Castolite Company was a retail wholesaler of a clear, liquid plastic (or polyester resin) called "Castolite," which was used for casting, embedding, laminating, molding, or coating. The company was founded in 1948 by William Weers (1906-1976). Initially, the scientific community used Castolite for embedding biological specimens into clear, liquid plastic, among other scientific applications. The process became popular in the arts and crafts community, and hobby shops frequently sold Castolite.

Weers was born Wilhelm Weers in Nordenhamm, Germany, to Mame and Johonna Loeschen Weers (dates unknown). He received his agricultural training in Germany. He then immigrated to America, arriving in New York City in 1925 at the age of nineteen. He worked as a florist, living in Evanston, Illinois. Weers became a naturalized citizen in 1931.

William Weers married Dorothy Thorne Weers (1909-2003) in 1938; the couple had two daughters. Dorothy Weers was a 1931 graduate of the University of Wisconsin and took a post-graduate course at Northwestern University. She worked as a medical laboratory technician and in the Athletics Department at Northwestern. At the time of their wedding, William Weers was working as a landscape designer for Weers and Seymour, a firm he co-owned with his friend, DeForrest Seymour (1903-1957), who was the best man at their wedding.

The couple moved to Woodstock, McHenry County, Illinois, and purchased a farm in 1948. The Weers lived on the farm where they ran a nursery and landscaping business in Kenilworth, and began a retail wholesale business selling plastics, the Castolite Company.

Upon William Weers's death in 1976, the Castolite Company (including a portion of the property) was sold to John Paul Kunzie (1935-2022). Dorothy Weers maintained the remainder of the estate, rebuilding the native prairie landscape and leaving the agricultural land in conservation easements.

John P. Kunzie purchased Castolite Co. in 1976. Kunzie grew up in DeKalb, Illinois, and graduated from Purdue University. After working for Johns Manville and R.R. Donnelley, a commercial roofing company, he founded John's Lab, a model railroading supply company. He lived in Woodstock, working as the proprietor of the Castolite Company from 1976 until his retirement in 2009.

Eager Polymers acquired Castolite from Kunzie in 2008 and incorporated its product lines and formulas under one umbrella (moving its headquarters). A large share of the Eager Polymers business was then sold to Smooth-On, Inc. in 2023; however, the sale did not include Castolite or its casting materials. Eager Polymers relaunched the Castolite brand in 2023.