Do you paint it fully assembled? How do you keep from getting it too thick around the safty, slide stop ect, ect and causeing trouble? I want to use high heat cerramic engine paint on a carry gun, it is a high heat, oven cured cerramic impregnated paint that I have found to be very sturdy.

You need to disassemble it.

What about the rails? Won't the weapon jam up if you get it on there? I'm concerned about the coating (whatever I use) increasing the thickness of parts and screwing up the tolerances.

It all depends on what paint you use. Some actualy advertise that it will act like a lube and is thin enough to paint every thing with. If you are going to do it strip everything down to the last part. If your sights are dovetailed I would remove them to but you could mask them off. Degrease everthing and when you think you are done degreasing DO IT AGAIN. I personaly would not try to paint my frame if it was polymer because I would be afraid I would screw it up degreasing it. I would also not bake the finish if your wifes oven for two reasons 1. She probably would not like it. 2. It is plain stupid and unhealthy you might not notice it now but 20yrs from now you might. So get the largest toaster oven you can find from a yard sale or Walmart if you are doing a pistol. If you are doing a rifle buy an old stove or something. I used Alumahyde 2 on my AK and it was been great and you do not have to bake Alumahyde 2. The down side is you need to let every thing set for a minimum of two weeks (NO TOUCHING OR PLAYING WITH IT!). I would let it set a month but two weeks should be good I am just one of those paranoid painters. If it looks dry enough in a few days to assemble do not do it or you will screw up the finsh somewhere. Brwnells has a good selection of stuff. If you have access to an oven that can be used I recomend the bake on stuff because you can assemble it sooner.
I hope this helps. They key to the finish is all in the PREP. Follow the instructions and it should go ok. I am planning on painting my blued Witness sooner or later.