Creating the Apollo Command Module thermal protection system was a labor of love. This video is from Textron Systems, once Avco Corporation (a conglomerate that has connections close to home, in the Midwest, to TV programs and stations it once owned).
This NASA film details how the CM’s ablative heat shielding was made. Fiberglass honeycomb was placed all around the spacecraft, top to bottom, custom fitted over every window, thruster, hatch and access port. Workers filled over 370,000 cells. Not a single cell was allowed to be imperfect. All of the ablative thermal-protection was cured and then machine-honed to thickness based on heating.
This film shows Avco’s work on the Block I Command Modules. These never flew manned as planned, of course. The Apollo 1 fire occurred inside the vehicle, killing its crew in a pad test, where Avco’s work could not aid them.
After the Fire, with little time to play with the Block I (the original Apollo designs had no Lunar Module and, thus, no docking tunnel), the remaining Block I vehicles were used in unmanned tests (often with Block II components for testing) and later scrapped or sent away eventually to museums.
Through this film and a little research, I discovered why the Block I CM was simply painted in that peculiar blue enamel paint, but the Block II had a silver skin.
The Block I didn’t have the Boost Protective Cover, so the ablator (with that blueish enamel paint) was thicker.
Adding the BPC saved some weight by reducing the CM’s overall ablator thickness underneath (as well as removing some excess internal insulation). Then, some Kapton coated with aluminum and silicon monoxide were laid in strips over the CM to provide improved thermal control while in space.
Enjoy the video.