I don't prime any of these figures, i find it unnecessary, all paints directly applied to the figure.
As for the arm mod itself, it's hard to explain, i mostly did it on a whim and with no real plan just kinda went with what i could manage as i broke it apart.
basically the solid gun arm was snapped apart, above the protrusion that has the tab for locking the barrel down in tank mode, this was what looked roughly like his elbow area, and where the new elbow was planted in.
Then i took off the flip down shield/blade assembly, i snapped the shield into 3 pieces, the two outside panels, and the inside section with tray for the blade. I removed the blade and snapped the inside section into two pieces and took off its inner edges so i could get just the small barrel looking sculpted plates. I then used those, and the two outside panels to build the big underside shape you see with the small spike (which was the two separate spike ends shaved and glued together). This required lots of shaving sanding and spacing up to get them to rough together into a shape then lots of superglue to get them all together and on the arm.
The blade was then glued on with the two swing arms for the flip down assembly slightly modified to fit onto a section of the gun arm which had small circles i shaved off, then everything glued in place so the sword is always out, prefer that and referencing one big pic i had of him it had that look, cannon in the middle, one look hooked blade and one shorter spike.
The ELBOW, was the ankle/foot from a protoform prime, i took the ball and the foot, used the ball and its peg into the upper arm with some slight modification and fitting, and the foot heavily carved down and plugged into the lower arm, so now he has a ball swivel elbow that rotates completely and goes up and down.
It was a bit complicated and as i said i didn't have a plan just started snapping the arm into pieces to see what i could get and what i could do, not something i'd recommend if your the least bit weary about messing it up, my concerns weren't huge i figured i could always just glue him back together the old way if i had too.
The arm is articulated in bot mode, the sound effects/light up with the shoulder movement still work, and i put him in tank mode and it works fine, he just has a big fat honkin end on his barrel so it looks a bit funky.