should'nt be going over 10% thinners 5% fine,the more thinner you add the more it speeds up the evaparation time which results in the paint dulling off,other factors for dulling off are
1 to fast a hardner
2 to higher a pressure on gun
3 spraying in cold
4 to much heat while spraying
5 soaking into primer
6 not allowing paint to flash of between coats
7 incorrect fan setting on gun
8 gun been held to far back from car
9 poor air movement
10 to high of grade sanding used on primer
11 to much material on car
12 use of aeorsal primer
i could go on and on and into detail but dont want to bore pants of you