I've had limited success with flattening clear. Most manufacturers will tell you NOT to do it. PPG for one. I did a bike awhile back, used a 4:1 mix and it didn't flatten hardly any (waiting 45 min between coats), used a 3:1 and it went somewhat, but not near enough, so I mixed a 2:1. Within about 5 minutes it was dead flat! Within 15 minutes, it was turning grey and cracking all over!!! Totally ruined the job and we had to take it all back down and start over $$$$$! AND, something about it, I'm guessing the parafins in it, makes it HORRIBLY hard to sand!!! Stuff is like a rock! Did a 32 couple a couple of weeks ago, started with a 3:1 mix and it didn't flatten enough, so being scared of what happened the last time, I did a 2-1/2:1 mix. Worked like a charm, but I ran out with just the rear quarter and deck lid left (2nd coat). Mixed another batch, 2-1/2:1 and sprayed it on the quarter and decklid and guess what, NO MATCH, didn't flatten enough... !!! . We sanded the whole car down the next day, went back to PPG (using all PPG products by the way) and bought their "Flex and Flat" clear set up. Its kind of expensive, but I will NEVER mix my own flat clear again!!!! The biggest problem seems to be inconsistancy of the flat. PLUS, if you ever have to go back and repair something, you probably WON'T get a match... Check out the Flex and Flat, or something pre-flattened, its a life saver!! Donny