There is alot to be considered if you are going to use clear coat. I would never clear coat over white. I have tried many times with different brands, and it always eventually yellows somewhat. Like supratuner said, sometimes it can end up to thick, especially testors "high gloss" enamel clear. Personally though, I`ve been able to get away with 5 coats of clear enamel, though they are thin coats and I heat the can with warm water first to thin it out a little. With duplicolor, I sometimes put 10-15 coats on, but it really goes on thin. Also, spraying laquers over enamels usually ends up as wasted effort.
I use it the same way I do colors: several thin coats followed by several heavier coats. Hope this helps.