To the best of my knowledge and based on my personal experience, both primers are enamel-based. Lacquers have always worked fine for me over either one. As far as which one would be preferable, Krylon Indoor/Outdoor is quite a bit smoother than Rustoleum automotive primer and easier to control (read less paint comes out, it builds up slower, and it lays down nicely without much texture). I have tried Rustolem Painter's Touch primer and this brand is pretty close to Krylon. The best thing about these brands is the price and availability.
Thanks man! So it is perfectly safe to use this primer and then spray Mr. Color lacquer on top of it? I thought lacquer ate through enamel.
Personally I always use a black undercoat with metallics. Gives them a nice depth IMO. Your mileage may vary, of course, and it depends how thick your metallic coat is. Some metallic paints (eg. Alclad) actually require a basecoat of gloss black, but I don't know about Mr Color. It couldn't hurt though!
Hey pyro, thanks for the replies. One more thing, but does a metallic painted piece still look good with flat coat on it?
Gold doesn't rust! It might tarnish a bit, but there shouldn't be any corrosion. I don't know where you are in the world, but if you can get hold of them, Citadel (Games Workshop) do some very nice metallics. They keep changing the names, but they used to do a "Shining Gold" that would match that tarnished look very well. EDIT: The new name is "Gehenna's Gold" apparently. Water-based acrylic, very nice to use, brushes on very well.
The musket looks simple - the parts that need to be gold are all raised, so should be easy to paint freehand. I wouldn't bother masking them. Basecoat of black, followed by the gold. I can't quite see from your linked pictures what bits on the head need to be gold, but it would depend on the part breakdown. Personally I'd try to keep masking to a minimum, but it's not always easy.
Hey pyro, so if I continue to use Mr. Color's brass, does the black undercoat still apply? It's technically metallic, so all metallic type paints need to have a black undercoat to make them more shiny correct?