Photo Etched Parts,Fastening to Model

Use one of three glues, 5 minute epoxy, crazy glue, or clear enamel.
Lee

The only fixative I use is super glue, or Mod Podge for attaching to clear parts.

demono69

Cyanoacrylate for sure. Thin glue for larger parts and thick followed by a dap of accelerator for smaller parts.

The only problem with superglue is it can sometimes leave a white residue. I know I suggested it, but I prefer to use epoxy or enamel.
Lee

It is true that the fumes from curing CA can cause crazing but when you get the hang of it, it's not a problem. I don't like epoxy because of the cure time and the waste. If you have to put a canopy mirror on for example there are two ways to avoid the crazing and the attachment is done in seconds. The first way would be a dab of liquid mask adjacent to the spot to be glued. I only use that trick if the area is very tight. Most of the time I have a small personal fan blowing across and away from the glue spot. I place the part, add a tiny amount of glue, and finish with tiny dab of accelerator. The tools I use for this are small sewing needles placed in dowel handles and with the very end of the needle eye cut off. I use nippers to break the end of the needle eye off, but protect your eyes that little piece of steel flies off quick!

or just prime and paint the part!

You'll get a better bond if you can glue metal to plastic. You'll end up with a weak join that could fail over time, with a metal to paint bond. That may not be an issue if you don't hold onto your built kits long. It is also dubious that priming will be used on a clear part. I personally don't prime anything, instead I rely on the preperation of the plastic. The exception would be an additional coat of paint over a heavily filled seam. My seams are invisible so I don't usually prime. Some prime, some don't.