You may either laugh or not believe this at all, but I've had good luck priming models with house primer. Yes - the stuff you use to prime walls, ceilings, exterior siding, etc... Specifically, Benjamin Moore's "Fresh Start " http://www.benjaminmoore.com/tds/1033/m7102300.pdf
I thin it about 50/50 with WW fluid (the usual airbrush consistency - "like milk") and it works quite well. One caveat - the specs on the can say specifically to *not* thin. Since I'm not trying to get 10yrs of durability on a sun-baked, rain-soaked wall of my house, my own opinion is that it's OK to thin. In anycase, the proof is in the pudding - it works for me! Before I tried this, I thought it might be too "coarse" to get smooth, even results from an airbrush - after all it was formulated for brushing and rolling. I was happy to see that when thinned, it airbrushes fine, giving nice, smooth, even coats. One of my modeling hobbies is model rockets. With this primer, the finish is so even that it perfectly "telegraphs" the fine spirals in the cardboard body tubes, and shows me where I need to do some more fine sanding. In this respect, it behaves just like a good automotive primer.
This particular primer is rated for use on metals, though I freely admit I mostly use it for plastic, carboard and other non-metallic surfaces. The few metal things I've used it on turned out fine, so I think it's generally going to work. It sands really easily, and makes a good base for all the acrylic paints I use. Sometimes I'll even tint it with some acrylic paint if I want to make top-coating go easier. If you don't use this particular brand, be sure to read the specs on the product you are using to see if it's suited for metal.
Make sure the metal is grease free (some use soap and water, others use alcohol or solvents.)
BTW, there seems to be a whole "movement" to using house paints for model airplanes.
http://www.modelairplanenews.com/how_to/latex1.asp
I would imagine that the amount of research and engineering in house paint technology is enormous. Joe/Jane Homeowner wants good results quickly, easily and wants it to last under some pretty brutal conditions. A side bonus is that you can get a quart of high-end house paint for under $10, a gallon for $35 or less. (Imagine what a quart of Tamiya acrylic would cost!) Personally, I just use what leftovers I have from house projects - so primer is essentially "free" for me.