I had to look up the definition for "Hydroscopy".
"Hydroscopy is the practice of viewing objects underwater".

- Tank under water
Herman surely meant Hygroscopy: "Hygroscopy is the ability of a substance to attract and hold water molecules from the surrounding environment. This is achieved through either absorption or adsorption with the absorbing or adsorbing material becoming physically changed somewhat, by an increase in volume, stickiness, or other physical characteristic of the material, as water molecules become suspended between the material's molecules in the process.
According to CollisionRepairMag.com: "Talc in body filler is hygroscopic, meaning it readily absorbs water. Don't leave a vehicle in damp conditions with exposed body filler. Moisture could get absorbed and give you big problems later in the repair."
Thanks Herman. I did not know that. No wonder the repair i did on my Ford Maverick 76 as a student did not last one Canadian winter.