MEKP (Methyl Ethyl Ketone Peroxide) is a catalyst used in the composites industry for polyester and vinyl ester resins and gel coats. It reacts with the resin to turn it from a liquid to a solid (cure it). It is used in varying ratios, generally use around 1%-3%, depending on the product being used.
Each True Composites resin product is available as a kit with a sufficient amount of MEKP. However, MEKP may be purchased individually, as shown here.