I would take the paint to the local jobber and have them re-shake it, and buy new reducer and hardener for it. Old reducer looses some of its properties from evaporation of the solvents, and old hardener just doesn't harden.
Try a bit of the paint on a scrap piece of primed metal before you bother the spray the truck to make sure it sprays right and sticks ok. Let it dry and make sure it stands up to a fingernail scratch test.
