The article you're looking for was in Street Rodder 2 or 3 years ago. It's in my locker at work so I don't have the issue on hand right now. I can find out in a day or two if you still need it.
Although the flattening agent works good, the main problem I've encountered with it is maintaing consistency of gloss. The agent settles very quickly in the can and when I've used it, no matter how carefully I stirred and measured the components, it always seemed like what I sprayed the day before had a "hair" difference in the gloss. Not drastic, just enough that you could tell a slight difference when set side to side. If you paint the whole truck in one shoot, it probably wouldn't matter.
I've also heard that the single stage urethanes in there nature try to gloss up. It takes quite a bit of the flattener to work, in the case of PPG it's usually at least 2 parts flattener to 3 parts paint, all depending on the gloss level desired. You have to get the P-Sheet from the jobber to know exactly what you're looking at, and then experiment to get the desired level of gloss.
I've heard it's easier to do BC/CC and flatten the clear but I've never tried it.
Bob
