Both polyester (Formula 27) and epoxy resins cure via an exothermic reation (they give off heat). The shape of the curing resin can affect the rate of cure. Spread in a thin film (lots of surface area), the heat dissipates and doesn't affect the rate that much. Contained in a cup (small surface area), the heat builds up and accelerates the reaction.

Formula 27 does go off quickly, but you can adjust the rate of cure by using less catylst (use 5 drops/oz vs. 10). Keep it out of the sun and spread thin on a plastic mixing palette. Once it stops spreading smoothly, toss it. It's done.

You cannot adjust the cure rate of epoxy by adjusting the resin/hardener ratio. You can use different hardners (205 / 206 for WEST System), but changing the ratio will give you a sticky mess that will never cure completely.