Have you ever seen a spacecraft being tested ?

Here is one: The ESA Venus Express. The reasons these guys have white gowns and hairnets is because you don't want to contaminate the spacecraft with particulate matters. Since the Venus Express is going to Venus, it is likely to feel very warm since it will be closer to the Sun during its mission. It is therefore imperative that any of the Multi-Layer Insulations cloth and other outside thermal protection materials properties do not degrade because of contamination. Any property changes of the outer layers will directly have an impact on the temperature inside the spacecraft.