The recommended levels of peroxides are 1.0 to 2.5 percent, with 1.25 to 2.0 percent being the preferred range. Situations where the low side of the range would be used include in hot shop conditions, for thick parts, with high resin content and for parts where keeping the exotherm down is important. Some examples where higher levels would be used are in cool shop conditions, for thin parts, where fast demold times are required and for composites with high filler and high glass content.
Conversely, going too high in peroxide level (typically over 2.5 percent for MEKP) can hurt the cure, too. The higher levels in these systems have more molecules that form free radicals; the number of free radicals present at any time is higher, and the higher concentration means that they are in close proximity to each other. This allows them to more easily react with other free radicals rather than the resin. When they react together they basically eliminate those free radicals from participating in the curing reaction. The free radicals also can react with the resin free radicals forming during the curing process, and that changes the crosslinked resin so it forms shorter chains. The shorter chains form a stiff composite. Even though it is hard when tested with a Barcol gauge, it is not cured well.
There are many variables to consider in trying to achieve the proper cure. If you have questions, work with your resin supplier to ensure the best results.