KL , You inspired me to get after my leaky Heater Core. I was dreading taking it back out of my truck. But last summer when it started to leak i install pex tubing ball valves in line of the heater hoses and put it on the back burner. It took me less than five minutes to remove it from the truck. probably just was putting it off because of past experiences with other vehicles and heater boxes being such a PITA. Luckily I had another heater box that came with my truck and that one tested good with no leaks. When taking apart the unit I had in the truck I also discovered mine to be a shade of blue green on the back side and inside the box that you speak of. Shined up nice with rubbing compound. Headed to my local Lowes. They had quite an assortment of hammered colored paints. Gonna see what I can find.
Jeff 1964 Chevrolet K10 Fleetside "A Kid's First Truck" Follow in the DITY 1951 Chevrolet 3100 Follow his build in the Project Journals 1969 Chevrolet Camaro Convertible | 2020 Chevrolet Silverado | 3500 Duramax | 2021 GMC Sierra Denali USAF Veteran 1983-1987 | PSP retired 1990- 2012
probably just was putting it off because of past experiences with other vehicles and heater boxes being such a PITA. 
