Dbu came out before the dbc I believe. Dbu is meant to be activated like spray said. You can activate the dbc also if wanted, but if activated any extra mixed can't be used at a later time. If you just add reducer and not activator to the dbc extra in the gun can be returned to the paint can. I heard ppg was trying to phaze out the dbu but too many people still use it, so it is still around. Check the tech sheets on ppg's website for what the products can be sprayed over and how to spray it, pot life, ect. Both have to be clearcoated. OOPs, realized I bascially repeated what spray said, didn't catch the first time that he said that activated it will harden up.