Sprayer Solenoid Internal Corrosion

I'm hoping someone has a quick fix for internal corrosion/rusting on sprayer solenoids. The problem:

 

My sprayer has four solenoids. One controls flow to a cluster nozzle. The other three provide individual shutoff for the right, center, and left spray booms. After each use of the sprayer, the magnetic internal parts of these solenoids rust/corrode and will lock up, requiring me to disassemble each solenoid, dry the internal parts, and remove the rust/corrosion using emery cloth. I've now begun to disassemble and dry each solenoid after each spraying job and spraying these internal parts with WD 40 during the reassembly process.

 

Is there a simple solution to this rust/corrosion problem that will make it unnecessary to disassemble these solenoids after each spray job.

Sprayer Solenoid Internal Corrosion