I have used Krylon Colormaxx Flat Crystal Clear (I buy it from my local Lowe's). You have to let the main coat COMPLETELY dry before applying it, otherwise it will peel like crazy. I would recommend a minimum of 2 to 4 days in a well ventilated area.
My custom USAF NW-2 (the booster unit for my Lionel TMCC Missile Train) was painted with Colormaxx paints and two very thin coats of Flat Crystal Clear to finish it off. The Krylon went on thin and smooth with very few runs.