Model Master Metalizers

For best results, you should not be undercoating withthese paints. THe surface should be sanded/polished as smooth as possible and the paint sprayed direclty on. The reason is that the paint is remarkably thin and will show the texture of the underlying primer base and you want it to replicate bare metal which should be completely smooth.
Applications to very small areas CAN be done with a brush, but it really is best to airbrush this. When doing so, you can reduce the pressure a bit since the paint is very thin and will come out a lot faster than hte typical enamel or acrylic.

Scott is right. I've sprayed them over paint, primer and bare plastic with no problems and the airbrush ready bottles cover and look the same as the rattle cans.
The buffing versions are intended to loose a bit during the buffing process; the non-buffing flavors seem to stay put so you should have better luck masking over those.
BTW - If you need a metallic finish for a car, try the Metalizers. Use a non-buffing version and with a coat of clear gloss the finish looks great. Make sure your bodywork is flawless though, any scratches will really stand out.

I use them quite a bit, but I use the buffing versions in rattle cans. I hate airbrushing laquer in my basement and with rattle cans I can take the parts outside to spray them.

I haven't found that any particular base works better than another. The colors I've used are completely opaque and the base color doesn't seem to matter. The airbrush versions may be different though.

The bad thing about them is that the surface is very touchy. It rubs off easily and never seems to really "Cure". Masking over it is always an iffy proposition. A coat of Future helps both those problems considerably, but it does reduce the sheen just slightly.