Understanding that it takes longer to flatten, what about the original "window" of the clear being used? On the 32 we did, we sprayed the 2nd coat and the next morning, it looked just like it did 20 minutes after spraying it... I use flattener in single stage at a 3:1 mix and never have a problem... Curious stuff! Now, anybody that is just starting to paint, stop reading right here!!! On the subject of mixing brands...Over the last 10 years, I have used several brands at once, mixing them here and there and have figured out within 99.9% what I can use with what else from another line. This is NOT saying something can't happen, because thru trial and error, stuff HAS happened, BAD stuff! But my experience has given me a chance to learn that some brands CAN work together in harmony. Doing CUSTOM paint, you find certain brands don't offer certain things or colors you might need, so what do you do? TRY IT! NOT on a customer job though. I've done sprayouts and baked them, put them in the sun, ran water on them and set them outside, for weeks at a time to test the results. Just cuz they look good today doesn't mean they will tomorrow!