Should I Prime before Testors Metallizer?

Probably would have been better to have it primes first - that is if you didn't prime yet. After the primer has dried and cured, spray a coat of put a coat of gloss black before using the buffing metallizer paint.

There's a combination there....not enough paint/too much buffing.

I spray directly to bare plastic. You will not get that metalic sheen over a primer (I have not tried gloss primer though...so..??). 

In the past, when I was using rattle can metalizer...I would "prime" a few select panels (color didn't seem to matter). Just as that was dry to the touch, I'd spray the metalizer. Those primed panels would have a different sheen, making them look another shade of metal.

For this one, you can see the difference on the wing. The darker looking panels were primed. The whole thing was sprayed with Aluminum Plate (buffing).

Nowadays, I keep several metalizers on hand, for airbrushing. Typically, I'll lay down a base of "Magnesium"....then tint panels with "Steel", Stainless Steel", Polished Alum", "Alum. Plate". (as needed)

My most recent NMF is a good example, with a lot of variety.

I agree with Fermis. I got to thinking last night and a thought did come to mind - perhaps it wasn't completly dried and cured before buffing? I don't know for I've never worked with buffing metallizer paint.

Phenomenal job on the BM aircraft. I've always wondered how you do different shade of bare metal panels on the aircraft. it looks accurate and real as the real thing. Well done!

I recently sprayed Metalizer (Buffing) Magnesium on the blade of my Meng D9 model. I cleaned the styrene with alchohol and then just sprayed raw on the bare styrene.  Let dry an hour then buffed.  It looks like real steel. This is great stuff.