Normally after the PCB is tested and ready to be put in service it is just cleaned with a reasonably hard brush and Carbon Tetra Chloride to remove excess flux and / or solder splatter. Then a normal coat of Varnish ( the stuff used for coating on motor or transformer windings ) would do.

This is a low cost option. But as members have suggested, a search in Google / your local market should give you some idea on commercial sprays that are packaged in aerosol containers.

But then again if you have a PCB with good solder mask use a good quality lead for soldering, a simple cleaning is all that is required IF the PCB will operate in normal temperatures and humidity levels.

Raghu