Yes DBU came out before DBC, I have been Spraying DBU since 1987or88 when it came out.
Back then if you let DBU dry an hour you could top coat it with lacquer clear, but hell i hated to rub , so I sprayed DAU75 Delglo clear over it and had great sucess with it.
I was spraying in a 75,000.00$ USItalia Downdraft paint booth , man that was like walking into the future after our old cross flow booths that just let dirt in like a windy desert.........LOL , Plus I made more money becaue i could paint more vehicles in a day because of the heat makeup in the booth, had a million BTU furnace on the back of it .
That was also before mixing banks ,and it was packaged in factory pack only.
I have been using PPG's TouchMix mix bank for around 14 years now and works great.
Now I use DBU and Top coat it with PPG's Global D893 clear as it rubbs like butter, much better then the 2002 or 2042, that stuff is like a rock after it has been cooked at 140deg. for 40 minutes .
Plus the global clear has almost no solvent pop problems. Its a more forgiving clear. Lays flatter too,