Some critical areas of the plane, such as the pilot-static system, must be taped off when the plane is washed. It has to be sealed so that no water gets into it. Then, when you're done washing the plane, you have to remember to take the tape off, or those instruments won't work. The underside of the plane also has to be cleaned, because it gets greasy from the plane's exhaust fumes. And then there are the bugs.
Insects often smack into car windshields, but the problem is worse in airplanes.
Once I flew into a swarm of locusts and I could hardly see out my windshield. Sullivan said that it took his crew of cadets about two hours to wash each plane. Dean Westermeyer, Aurora, Mo., agreed that cleaning a plane is hard work. By the time 90 minutes had passed, I had only cleaned halfway down the belly. Westermeyer added that he was pleased with the good job done by the cadets, and just hoped he could keep his plane sparkling clean for a while.