applying decals, a query !!!!

Hya all, i.ve bought a 'sanger' 1/48 vac form avro lincoln,   on the instructions about putting the decals on model it says YOU MUST USE WATER OR ACRYLIC BASED LACQUER TO VARNISH, NOT POLYURETHANE OR ENAMEL LACQUERS !!! over the decals, does anybody know why you can't use enamel paints [will it attack decals] and what are the above lacquers like to put on model, never used any water based paints at all............ any replies welcome  ....... ian.