Fire alarm control panel got wet from a leaking roof. Water entered all the circuit boards but the system was not energized. Already developing rust in some areas on the boards; how do I clean and dry it completely before I power it on to test it?

I would suggest natural drying or you could use a hair dryer on the lowest heat setting. You can purchase switch cleaning fluid and clean it with a stiff brush, may work, but it is more likely you will have to replace it.

As its a life safety system and is showing signs of corrosion I would not try and repair it since if it has any micro electronics they will have been destroyed. Expensive that it will be I would change everything that got wet, that way it will be 100% reliable