Will enamel thinner eat a layer of lacquer?

If the lacquer is fully dried, then you shouldn't have any problems.  Just make sure to use enamel thinner in the wash and not lacquer thinner or mineral spirits.  However, I'm not sure that mineral spirits will affect the lacquer, you may want to test this on a scrap piece of sprue.

Thanks for the quick reply! Don't worry: I'll use enamel thinner. I don't even know what mineral spirits are, let alone have some... Haha!

Actually, any organic solvent may affect hardened lacquer. To be safe, apply an acrylic clear over the lacquer, then apply your wash.

Sure. Thanks!