The hardener is a catalyst that make your paint cure or "harden", if you didn't use it your cars paint would never properly dry. Reducer is to reduce the viscosity of your paint. Not many spray guns can handle thick paint without it coming out like "spider webbing". Reducer is to reduce the viscosity of the paint or to make it thinner so it can be applied as smoothly as possible. Its also used to clean paint off of things after application.
Its also good when you didn't buy enough paint so you reduce it down to cover, hehe
It's always good to use the same products from start to finish when dealing with chemicals . Like when using Dupont paint use Dupont reducer and hardener catalyst. Better top be safe then sorry,especially when 250.00 a gallon and up paint!!!!!!!!!!!! Good Luck.
Here's a tip; I use the hardners in ALL my enamel paint. Testors little bottles, model spray paint (take it out of the can and use it in an air brush), everything. Does the same wonders for those paints that it does for the fancy car paints. Of course test a little first. Be my luck that it didn't work and ruined your $$$$$$ project and you would put a contact out on me. It has worked in all the paints I have tried it on.
Good advice though to use brand name stuff in the high buck auto paint, but do use it - it is great stuff.
You could also go with the 4:1:1 acrylic urethane combo with Centari. If you look at the spec sheet the paint gets higher ratings mixed like this vs. spraying it as is. Basically they add the urethane elastomere to the reducer and call it "Performance Pak." If you do it that way the cans you get should say "Centari Ultra." It doesn't cost that much extra either.