68GT: Polyscale is out of production and will probably be hard to find. Most railroad shops I've been in carry Floquil products, if you can't find one near you I'm not sure where else to look other than on-line.
I like the Metalizer, works well as long as you don't try to mask over it. Frankly I use Alclad for pretty much all metallic surfaces these days.
Clemens: Good luck with NM GB V - I'd like to support but I'm swamped by stuff now and after building three P-47s and a P-51 need a little break from NM subjects! I was pretty new to the whole GB thing too when I started my two GBs so if you need any advice give me a yell.