5 Star Synthetic Enamel Hardener 5132. Other people have had good luck with normal acrylic enamel hardener, but I wanted to use something that was designed to mix with alkyd enamels like Rustoleum. I mixed it 8 parts paint to 1 part hardener and then let that sit 15 minutes (sweat-in time) before final thinning with the mineral spirits and reducer.