Why do I need to add MEKP to polyester resin or gelcoat? How much do I add?

Polyester resins and gelcoats require the addition of MEKP (Methyl Ethyl Ketone Peroxide) catalyst to cure in a desired amount of time. If these products were left to sit on a shelf for a long period of time (many months or even years) they would eventually cure on their own. MEKP speeds up the curing reaction of the polyester resin in these products for a specific working time. This reaction is also dependent on temperature.

The amount of MEKP depends on the specific brand and product, but, generally, is 1-3% for most applications. 3% would give the shortest working time, but might be necessary for cooler conditions. 1% is commonly deemed as the safest amount to use for the longest working time, or when the user is not as experienced. It's okay to add less than 1% for larger layups or applications, because doing so will extend working time.

Most polyester resin products will either come with MEKP or will specify how much MEKP should be used. For example, TotalBoat brand polyester resin and gelcoat products indicate how much MEKP to add per ounce of product.