Mastering the Chemistry

For Polyester and Vinylester resins, the «hardener» is actually a catalyst called MEKP (Methyl Ethyl Ketone Peroxide). Unlike epoxy (which has a fixed ratio like 2:1), MEKP is added in small percentages (1% to 2%) to initiate the cure based on environmental conditions.

Calculating the Ratio

The standard range is 1.5% to 2% by weight.

# Example Calculation
            Resin_Weight = 1000  # grams (1 kg)
            Target_Percentage = 0.015  # 1.5%

            Catalyst_Needed = Resin_Weight * Target_Percentage
            # Result: 15 grams (or approx 15 ml) of MEKP

Rule of Thumb: At 25°C (77°F), use 1.5%. If it’s colder, increase to 2%. If it’s very hot, reduce to 1%.

Consequences of Improper Mixing

Safety Note: MEKP is an organic peroxide. It is extremely dangerous to eyes. Never mix MEKP directly with Cobalt promoter (an accelerator often pre-mixed in resin) as it can explode. Since most resins are «pre-promoted,» just add MEKP to the resin.