There are many reasons to take a perfectly handsome rifle and go 'an mess it up by pouring multicolored splotches all over its exterior. But the primary reasons are: 1, camouflage/concealment; 2, you can't leave well enough alone; and 3, you can't afford to have it professionally finished. I for one fall into all those categories, but mainly #2.
Disclaimer: I am not a duty slotted sniper. I am not a duty slotted paint-and-body man either. Due to some encouragement from some SC hogs I am, however, training to become a Gunsmith. In the process I have tinkered with all of my own personal weapons, which includes painting my Remington 700 VSSFP (could they have more d%$n letters?) for the fifth time now. I don't claim that this process is the only way, nor even the best. I am just passing on what has worked well for me in the hopes that someone else who fits the above 3 categories will get a little out of it.
This is very similar to reloading in that you can be as meticulous or as sloppy as you want, and you can spend from a few dollars to a lot.
I will offer this in a few basic steps.
I degrease all of the surfaces that I want primer and paint to adhere to by first using acetone, drying, then using alcohol (the non-drinking type), and finally washing with soap and water.
Use gloves or clean towels when handling the cleaned rifle.
Anywhere you don't want paint, use masking tape. For larger or odd shaped areas use aluminum foil, then tape the edges.
In the past I have always used a zinc oxide primer and either Krylon or Rust-oleum flat paints. Both brands have worked well for me.
(The next paint job will be with Brownell's lacquer paint.)
First lay a coat of primer, waiting a full day for it to dry. Then, after the entire area is covered and dry, apply the paint, waiting for it to dry well before handling.
That's pretty much all there is to it. Nope, it's not rocket science. It's not magic. It's not a $150 professional hi-tech coating. But it is mine and I did it my way. (Don't make me start singing.)