You're right, they are lacquers, but are designed to be sprayed directly on bare plastic without prior thinning. Be careful, though:
1. You can't handle/touch the part you sprayed without leaving a fingerprint (or worse).
2. Spraying a clear overcoat to protect against #1 above tends to reduce the effect you tried so hard to produce.
3. The underlying plastic has to be completely free of any flaws (scratches, etc.) or the Metalizer will enhance the mistakes.
Other than that, Metalizer is a pretty good product (IMO). I personally like using it on the smaller "accessory" metallic parts (like jet exhausts). If you are going for a complete natural metal finish (like an aircraft), you might want to consider the more durable alternatives like Alclad or SNJ.
Hope this helps some.
Gip Winecoff