Wash the truck body thoroughly with dishwashing detergent and water. Scrub the truck with a sponge to remove all loose surface debris. Rinse the truck, and allow the truck to dry completely before continuing.
Wipe the truck body with mineral spirits to remove any grease or wax deposits. Use paper towels and nitrile gloves to apply the mineral spirits.
Sand any glossy spots on the old paint job with fine-grit sandpaper. Sand the old finish until you have removed the gloss. Rinse the truck and allow it to dry thoroughly.
Move the truck to an enclosed, ventilated area to protect it from airborne contaminants, particularly dust, during painting. Check that the painting area is absolutely dry before painting---moisture in the painting area will interfere with the ability of the urethane paint to properly cure.
Mask off the wheels, windshield and any other areas of the truck you do not want to paint, using painter's plastic and masking tape.
Put on a respirator, and prime the truck with a thin coat of urethane auto primer. Slowly squeeze the trigger of the airless paint sprayer until the primer is leaving the nozzle in a thick mist. Prime the truck by moving the nozzle of the paint sprayer gently back and forth until you have evenly coated the truck body. Allow the primer to dry for 90 minutes.
Apply the first base coat of urethane auto paint with the airless paint sprayer, spraying a fine mist onto the truck to create a very thin initial base layer. Allow the initial base layer to dry for 15 minutes, and then apply a second layer. Continue adding thin base layers until the primer is no longer visible, waiting 15 minutes between each base layer application. Allow the paint to dry for 60-90 minutes after the final layer is applied.
Apply three coats of clear urethane auto paint to the truck. Wait 15 minutes between clear-coat applications, and allow the truck to sit overnight after the third coat is applied.