The assertion that operational U-boats were regularly overhauled and repainted is supported in a recorded conversation between German sailors in captivity, as published in Black May by Michael Gannon (Aurum Press, 1998) –
Radioman from the surface tanker Germania: Is a boat [U-boat] painted each time it sails?
Spitz [U-boat sailor]: Yes, scraped and repainted.
Kalisch [Second U-boat sailor]: Strelow’s boat [U 435] once came back entirely covered with rust; he had been out for twelve or thirteen weeks. The whole boat was a reddish-brown.
(extract from "Kriegsmarine U-Boat Colours & Markings" by Dougie Martindale)