Too late but I try and talk people out of doing this. Rustoleum is oil based, not acrylic or urethane so using those type of hardeners do absolutely nothing. Play mad chemist but there's no urethane in it for urethane hardener to harden. Rustoleum as an oil based paint falls into the alkyd family. Water towers, park benches, and fire hydrants. A paint family that's main purpose is protection over appearance. It's a softer more resilient abrasion resistant finish then catalyzed acrylic or urethane paints. The shines lasts about as long as it takes to dry. 60 days in.
The real problem can arise when you attempt to put a true automotive grade clear coat on top of the uncatalyzed OIL-Based disaster where it can wrinkle up, lift, fisheye no matter how many times you wipe it down prior to re-paint etc.