Metallizers (like the Testor's brand) are an organic solvent-based paint with very fine metal particulates instead of colored pigment like regular paint. Once sprayed on, some of them can be buffed (small piece of tissue, cotton swab, etc) to a quite realistic metallic shine. As I understand it, the rubbing action during buffing causes the small metal particles to sort of "lie down and line-up" with each other. One of the requirements of metallizers is that the substrate (usually the plastic) has to be ultra-smooth, or the paint will pick up and highlight such things as coarse sanding gouges, seams, glue spots, etc. The paint itself is very thin and designed to be applied with an airbrush right out of the bottle with no thinning, although I have applied it with a "regular'" brush to 1/35 small arms gun barrels with pretty good results. You can't (or at least I can't) mask it with tape, and any metallizer sealer you put over the finish tends to diminish the metallic effect you are trying to achieve. There are other metallizing products out there besides the Testor's brand, each with their own advantages/disadvantages. Ask any of the regular posters here, and you'll get as many opinions on which is best.. The important thing is to try a couple for yourself. Experiment, and see which one produces the best finish that you like. There are other threads throughout this forum dealing with metallizers (decaling, etc). Give it a search and see if you can find an answer to your question. If you can't, we're here...
Gip Winecoff