Handpainting or airbrushing ? If you are handpainting,Future floor polish is a tough acrylic sealer which hand brushes well because of its leveling properties.You can also use a rattlecan clear gloss if it's big areas.For airbrushing,Testors,Modelmaster,and Vallejo make Clearcoat for airbrushing.
Drying times depend on what you use.
Also further down you have the Decaling Section or the Painting Section for this type of question.