I use HPA acrylic spray which is 'conformal coating' and available from Farnell in most countries.
Funny thing is a have an aerosol which had been on the shelf for several years, I just finished assembling a rather complicated PCB which will be used in a damp environment so I thought I would coat it for protection. The can had 'gone bad' although the HPA itself was fine. The nozzle blew out and a single stream of acrylic shot all over the board and flooded it :x It isn't only damp proof now, it's entombed in plastic! It's a good job I sealed all the connectors before starting.
Brian.