bare metal finishes

Aclad - enough as been said.. I use it and love it..

BMF - great for masking canopies... great if done right on an aircraft but if you mess up it could be expensive.. ($7 a sheet or so)

Testors metallizer - requires a sealer as it is delicate and can rub off without it.  On the flip side some of the colors give a remarkable look (burnt metal pops into mind for radial engine cylinders)

Out of the 3, I would go Alclad in a second...

The only disadvantages I can come up with are the poor alclad gloss black primer which I won't be using again (I will use future), the lacquer fumes, and the absolutely clean model surface you need to apply the base coat and the alclad.  Tiny dust and lint and imperfections show up rather easily through the alclad.

There has been quite a bit posted lately on the Alclad gloss black undercoat.  For once I seem to be the lucky one...never a problem with it except the time I used lacquer thinner.  Alclad uses MEK solvent, which doesn't play well with lacquer thinner.

If you use Alclad 2 Highly Polished Aluminum or Chrome, you'll need some kind of gloss black undercoat or you won't get the reflective effect.  I've used Alclad 2 Gloss Black Undercoat, Tamiya spray can Black Lacquer, Scalecoat Locomotive Black, all with good results.  A coat of Future over gloss black enamel or acrylic should act as a barrier and keep the MEK from attacking the base coat and crazing the finish.

And it can't be emphasized enough: surface prep is critical.  the surface must be about perfectly smooth, and free of dust, oil or other contaminants.

I just finished doing my first bare metal finish with alclad.  It was very easy.  Use future as a base coat, then apply alclad over it.  After 24 hours of dry time, I've masked over the alclad with no ill effects and I've painted acrylic right over it with no ill effects.  It looks great.

The only disadvantages I can come up with are the poor alclad gloss black primer which I won't be using again (I will use future), the lacquer fumes, and the absolutely clean model surface you need to apply the base coat and the alclad.  Tiny dust and lint and imperfections show up rather easily through the alclad.