Urethane paint contains isocyanate of some type. That isocyanate reacts with either water (humidity in the air - single component urethane) or a polyol (2-components paint part A/part B).

You can dilute down a urethane paint (either type) with reducer, but make SURE the reducer doesn't have any alcohol in it (any chemical that ends in "ol"). Toluene or xylene should work for you. Go ahead and give it a shot.