Are we still protecting pcbs with wax?
That substance is what is known as 'conformal coat'. It is used to protect the traces and connections from moisture. 

If this is an ionization-style detector, there are nodes within the circuit that operate at extremely-high impedance. The conformal coat protects those nodes from contamination. 

There are many different conformal coat products available. I've used a product similar to what you describe in the past. 

GE and Dow Corning make the conformal coat products that we currently use.