The concern, as Jepp2's 4350 experience exemplifies, is that when the nitrocellulose breaks down, the acid radicals that make both red fumes and the red dust are destroying the powder's deterrent coatings faster than the nitrocellulose itself. That results in the powder's burn rate going up. Sometimes way up. Such powder should not even be experimented with, IMHO. It costs less than a gun does to replace, so one is better off getting some new powder.
I would be wary of red rust that came from an old steel can. You have to ask why the can rusted. If it did so from the outside, you may be ok. But if it did so from the inside, the chances are the rust was started by acid fumes coming from decomposing powder.
Powder does not start to decompose until its stabilizer is consumed. Once that happens the burn rate can go, or the powder can weaken. Chances are a lot of it does both, in sequence, first getting dangerously fast, then fast but weak.