Reducers and hardeners must be mixed in exactly as directed in the paint manufacturer's instructions, or the consequences can be catastrophic. Their concentrations must be adjusted for changes in temperature and humidity as well. Straight out of the can, paint is like a syrup and must be thinned before it can be sprayed. Hardeners, on the other hand, are mixed into primers and clears to assist with curing. The exception is water-based paints, which are ready to spray right out of the bottle. Unlike urethane paints, water-based paints are nontoxic, so you can spray them without a booth. Some industry insiders believe that increasingly stringent state laws may someday phase out urethane paints in favor of their environmentally friendlier water-based counterparts.