3 gallons would probably do the M109. Be smart and roll the top of the box, it will never show, and you will lose a lot less than when you spray. A conventional high pressure spray gun has a transfer efficiency (TE) of about 25%. That means that 25% of the paint is on the vehicle, the rest is overspray. HVLP (High volume, low pressure) guns have a TE of 45-55%. If you know someone who paints wrought iron fences, he might have an electrostatic HVLP gun, it will do about 75% TE. The e-stat gun puts a high voltage (30-60,000 volts) at extremely low amperage (20 microamps) charge on the paint, causing it to be drawn to the metal of the truck like a magnet. The only problem with E-stat guns is the price, $3500-$6000 depending on the brand and model.
This might sound like a lot of hooey, after all, what's a little paint? I spent $2.4 Million last year on paint at Honda Lock, and I am on track to spend $4.3 million this year. We hunt for 1 and 2% TE increases wherever we can find them! Some of our colors run $280 a gallon and up for the base coats, the catalyst is $110 a gallon. This is in 200 gallon lots!
I am lucky, in my line of work I have accumulated all kinds of paint equipment over the years. I use a 1/2 gallon pressure pot and an HVLP gun for most of my painting. My e-stat gun is great when you are painting the undercarriage of a vehicle, because the paint wraps around brake lines, bolts, etc. This makes getting good coverage much easier.
When I get my shop set up, I will be able to do a MV paint job in a long day. The offer to use the shop and equipment will be made, if anyone wants to drive to Carrollton Ga. I'll spray, you help, my wife will cook. We can swap lies!