Don't use WD40, it is not intended to be an electronics cleaner, it removes water, lubricates and leaves a residue.
Here is how I did it with a really filthy SX-780.
Remove wood covers and other likely to be damaged-by-water parts.
Take chassis or boards outside, and set on hard surface so liquids can drain away.
Saturate with Simple Green and let it soak for a while.
Rinse it with water, you can brush with a soft brush if needed. Be careful not to flood it, just rinse and drain.
Repeat Simple Green and rinse/brush treatment if needed.
Blow away water with compressed air or similar, gently, don't blast it real hard, it is not necessary.
Rinse down well with isopropol (99%) or denatured alcohol to remove the last of the water.
Blow dry gently again.
If it has a receiver with mechanical tuning capacitor rinse the fins with a
non-residue electronic cleaner.
Let it sit in a warm place overnight, to finish drying and alcohol evaporation, A warm incandescent light bulb and/or fan helps.
Deoxit switches and controls with appropriate type.
SX-780 before clean:
View attachment 1349875 View attachment 1349877
SX-780 after clean:
View attachment 1349879