Throughout the long course of my drywall repair project I have certainly learned to only use metal putty knives to apply mud. A wider blade is better, not to mention the fact that the whole thing seems to go better for different reasons such as better spreading and etc. That said the metal knives all seem to rust quickly.
I have began resurfacing my most often used knives with a crystal clear enamel coat in a rattle can. This seems to be keeping the rust away, at least for some time.
QUALITY CONTROL NOTE
Make sure the coating goes on smoothly. Drips or bumps will show up in your mud, no matter how small.
Due to the fact that I regularly scrape dried mud off of my knives, the enamel coat is not permanent. Below is a picture of one that had endured 2/3 months of use and abuse. The majority of that time was sitting with mud on it and then being scraped to use after a few days, whereas being used every day and kept clean would be better for it. Anyway, if I can sand off the old coat and redo it once in awhile, it is worth it to keep rust and mud from mixing.