Power Transformers - Rust. How much is too much?

[Alley_Cat_Jack]

I picked up some junk to pick salvage off about a year ago and had these transformers among them. I would have thought they were scrap, but recently I saw some mentioning that rust can be okay to an extent on a transformer and in some cases is even sought after. What I have not found is however how much is acceptable before the transformers will break down.

Here are the pair of Triad R-20B I have:

https://imgur.com/a/TcEb5

[Enzo]

Exterior rust on the end bells is unpleasing to look at but won;t hurt the transformer.

[Sowhat]

They look good to me, just a little wire brush action and a squirt or two of gray Rustoleum and they will be as good as new... providing that is if they don't have shorted or open windings somewhere.