I wasn't aware that POR15 was a high temp coating. I guess its not too surprising as it can withstand sandblasting to some degree. I have always used high temp silver paint under the blue for the same reason. The rust underneath the inevitable paint peel looks poor otherwise for sure.
Did you find your blue yet? I believe I have some 78 blue valve covers boxed up somewhere, I need to check..... Unfortunately my Y88 survivor cars engine has been touched up with an incorrect blue. Can't blame anyone but myself for that. When I bought the car, I detailed the engine compartment. Wish I wouldn't have now, but at the time it seemed like a no brainer to do. I also threw away the original spark plug wires & took zero pictures......
Never thought I would care years later.