Conformal clear coating is available from most electronics stores for the purpose of providing a protective sealing layer against moisture, chemicals, dust etc. This is an essential protective barrier for products sold into industrial and oil and gas sectors where protection is needed against H2S in the air. H2S eats copper traces rapidly. This clear coating is available from many electronics specialty stores but if not applied by the board manufacturer before sale, may void the warrany on the basis of:
a) Reduced PCB heat dissipation (shortens board life)
b) renders board component replacement/repair impossible.
But once you are past warranty, it could extend board life by eliminating contaminants as mentioned previously.