Owen_Dyneto wrote:
Shouldn't be any rust on the heat exchanger itself, it's stainless steel. Could be some scale I suppose from neglectful prior owners but a simple visual inspection should reveal that. Trying to save the mild steel outside casing by coating it is a good thought but since you won't be able to keep the coating off the heat exchanger itself, I think that in the final analysis it's probably not a good idea.

There is I believe a reason why we so often find the necks badly corroded and sometimes perforated, other than that Packard didn't consider their need to last a half-century. I think it is exacerbated by the location of the hose clamps. If the clamp is well down on the hose and neck towards the cooler body, the hose will bellow a bit from the clamp tension and there remains a static cavity for coolant to lay dormant and once the corrosion inhibitor is exhausted (2 years?) you can get attack of the neck from the outside above the clamp location (which is the common observation). At least that's my theory. I think the clamp should be as near the end of the neck as practical. And since this area is the most vulnerable, I think your idea of using POR is a good one, but I'd limit it to the inside and outside of the two water necks.