Just my opinion, but if the lacquer is gloss or semi-gloss, then a prime coat would be highly recommended. In addition, if the substrate (the plastic) is a dark color, and your lacquer topcoat is light or has pronounced contrast (blue, red, etc.), a light colored primer will give you truer coloration. A primer is also recommended (almost required) if your topcoat is going to contain any gloss white or yellow. Floquil's Reefer White is an excellent primer, as are many of the lighter grays made by Testors.
Hope this helps some.
Gip Winecoff