I actually, after printing, spray the sheets with el cheapo Walmart Matte Spray Lacquer at about 2$ a can,
I think CrystalClear is the brand. I spray one side with just enough that the card tends to become translucent when dry flip and repeat. The actual painting is either by brush or airbrush, depending on the effect I want, smooth or grunge. Grunge calls for a combo of the two, base coat with an airbrush or spray can ( The Cameo stuff from Walmart, though expensive, does a great job, and as I only do a small amount, the cans last a long time... all the other paint I use is mixed from liquid acrylic or tube acrylics, and matte acrylic permanent varnish, with a touch of water or just the right amount for airbrush work. The varnish also comes in Gloss by the way, but I very rarely use it in military modeling. I want to try Dulcoat one day and see if it is something I can use or not.
Best thing is an airbrush, though paints are EXPENSIVE to buy, and mixing is a pain! Spray cans are taking a gamble every time as they can suddenly sploosh out globs on your model, and then you are screwed. It's a 50/50 risk.
Brush... only if you want grunge as there is NO WAY to get a really clean smooth model by painting with a brush! You can get close, but not as well as an airbrush can do! There are all sorts of trials and tribulations in the Painting realm! Painters tape for masking, one time won't stick and you get overspray bleed, next time it rips the top surface of the painted part! Ya never know!
I would NOT recommend an actual PRIMER coat, with PRIMER! It is TOO thick unless you decant into a jar and thin to needed consistency ( talking about acrylic here) auto primer is always thinned at least 2-1 but lacquer based.
Actual Gesso is also too thick unless thinned, I don't care for the acrylic Gesso that is out now, too plastic for me.
For a better answer, it would depend on the model. Your plan for spray paint sounds OK, just depends on if it needs matte finish or gloss and what type of primer coat. If you just need to seal the model before painting, the Matte Spray Acrylic Lacquer might be a good way to go. I assume you are talking about the Chimera? Then the Cameo series Walmart brand might be a way to go, as it would lay down a nice even flat primer in "?" military color for you. Then you could weather and dry brush with your other acrylics!
You could try the flat black Interior/exterior Fast Dry Walmart special see this...
http://www.zealot.com/threads/wont-say-what-it-is-till-later.180161/page-3#post-1026058
makes a good base for painting and weathering, and there several ways to play with it. You can lightly rub it with an old piece of denim and it will make it look like a worn tire, with a burnisher you can make it look like metal with almost a dry brushed metal edge.