Thanks to the magic of Mr. Hobby paints and Matte Spray, a relatively simple custom turned out better than expected. I'd intended this just for my shelf, but wanted to share how it turned out.
The matte light reflection effect (similar to FrogDNA's TF cartoon-styled customs) really come across well thanks to "Mr. Hobby UV cut Super Clear Matte spray". It's replaced my use of Citadel matte spray permanently. Really nice stuff! It also makes the figure feel really good in-hand, and there's no paint or sealant rub even after repeated transformations.
The reds/blues are actually just the factory plastic color with panel lining. Silvers/metallics are Mr. Hobby Super Metallic Chrome Silver, and in some areas that mixed with their glossy black for a darker gunmetal effect (legs, hands, biceps). I removed the original faction symbol with Mr. Hobby thinner and Q-Tips.
Prime's head was the extra one (without the screw-assembly) provided with MakeToys Battle Tanker - hence the 'G2' in the title. I didn't have any G2 faction symbols, so I went with the smaller reprolabel G1 metallic ones.
Modified the torso a slight bit to allow clearance for the larger head, and near the torso balljoint for better mobility.
Weapons are from the Infinity War II set (Junkion Blacksmith), and the gun game from a TF Prime FE figure. The small sword in the larger assembly (I believe) was an Energon figure's weapon. I'd love for some identification on that piece though - it'd be fun to source the same sword out in different colors.
Hope you like the pics.