Couple of other things.

If you are using an air brush or detail gun, start out at 20 psi. If it won't spray properly, increase to 25 psi. If it still doesn't spray properly, the paint most likely needs additional thinning. You should be able to spray latex at 20 or 25 psi. You do not need to (or should) use a higher pressure.

A helpful hint - I add 1 - 1 1/2 caps of Flotrol per quart of UNDILUTED paint. The Flotrol retards drying somewhat, thus allowing any sags, etc to "level" themselves out.

Make sure the latex is completely cured BEFORE clear coating it. Press your thumb on a firm area, if it leaves a thumb print, it isn't cured (don't worry, the thumb print will disappear). Many latex paints cure in 7 - 10 days, but can take as long as 30 days.

As a personal preference, I like and use MinWax Polycrylic for a clear coat. It is a water base polyurethane that doesn't yellow. ACRYLIC lacquer, Ultracote clear and Lustercote clear are some other good choices for clear coating also.