Think of reducer as paint thinner. VOC stands for Volatile Organic Compounds. That's the nasty stuff (isocyanates) that'll rot yer brain and nervous system if you breathe it in. Some places require low or no VOC paints. If you're in one of those areas your local paint store will know. When you see somebody do something really stupid, or crazy, and you say he's having an "isocyanate moment", if anybody knows what you're talking about....he's probably a painter.
The new urethane paints don't so much "dry", as they outgas. The solvents evaporate to a point where only the solids (pigment) are left behind. That's why recoat and cure times are so important.
The hardener is also sometimes called the catalyst or activator. It's mixed in with the color and reducer at the time of spraying. Otherwise the paint would harden in the can. Some companies, House of Kolor for one, pack thier paint under a nitrogen "blanket" to keep em fresh. If you're paint comes packed like this, don't crack the seal until its time to spray. It's hard to beat back the temptation I know. But the paint can start to deteriorate when its exposed to air for too long. It's why catalyzed paints are so much more durable than say, rattle cans which have to remain under pressure to stay liquid. Slow, med, and fast reducers have already been covered. I think that's everything, if you have any more questions feel free to ask...
