Ultrasonic Cleaning Solution
Brief partial answer.

Water is an excellent start - a highly polar solvent - it just happens we are mostly made from it and don't realise how effective it is compared to some alternatives.

Small quantities of mild detergent help. After that there are many specialist materials you can try - but theabove will get you gping. 

Ensure that all components are water safe. 
eg some sounders, relays etc may not like immersion.
 Some components have a stick on 'label' intended for removal after immersion cleaning.

Some few components may be damaged by mechanical issues - eg just maybe very sensitive crystals. Probably rare, but be aware.