I've never used ball joints or any other kind of joints before and I was wondering what others would recommend for use on a HGUC gundam.
Articulation in:
feet and ankles
Hips
Shoulders
elbows
waist and neck
For some of this I gotta ask what you're hoping to accomplish. The HGUC Gundam already has good posability in the shoulders and ankles, for instance. How much farther do you want to push it? This goes beyond simple "joint replacement" and into the realm of creating more elaborate joint systems, usually what I refer to as "self-extending joints" which reposition themselves as they turn in order to give the parts more clearance to move. Often the problem isn't a matter of getting adequately posable joints, but finding someplace for all those bulky armor parts to move to and still look cool. But anyway...
For elbows in 1:144 kits I highly recommend two parts: the first is the Kotobukiya Mecha Joint (small), the second is the Wave T-Shaft #1. Both allow you to create a simple, small, double-hinge joint. The T-Shaft is more suitable for the job out-of-box, it forms a nice little plastic (ABS? Styrene?) hinge block for the elbow with two (poly-vinyl) struts for mounting into other parts. The Kotobukiya part is larger (but can be easily trimmed down) - the whole thing is made of poly-vinyl, too, so it's unpaintable unless you case it up in something. The Kotobukiya part also has much better load-bearing capacity - it's a high-friction joint. The Wave part is adequate to hold my Wing Zero's beam rifles, but in order to get it to perform that well I needed to clamp the plastic parts of the hinge until the glue dried.
My Strike: You can see the elbow joint part in the left arm, it's a Kotobukiya Mecha Joint that was trimmed down and then boxed up in styrene to make it paintable.
My Wing Zero: It's hard to see the elbow joints in this photo: unfortunately I don't think I have a better photo of that part of the model. The elbows here are Wave T-Shaft, the only change I made to the parts was to bulk up the back side of the elbow with some styrene.
Those parts are great for replacing elbows in 1:144 kits - but there is another problem you must face. Most 1:144 kits these days combine the elbow-swivel and the elbow-bend into one joint, usually a polycap mounted in the forearm. If you replace your model's elbow joint with one of the above parts, you'll need to replace (or fix-pose) the arm's swivel-joint between the shoulder and bicep. Usually this means installing a new strut in the shoulder and a matching polycap in the bicep part.
As for ball joints: they're sort of a tricky business in some ways... The type with a polycap socket generally won't be very good at holding a pose, it's too hard to get adequate friction with them. The strongest ball joints you can buy are the Yellow Submarine "Kansetsu Waza" ball joints. They're made out of a clear acrylic material, like CD jewel cases or various transparent ballpoint pens. Apart from being great for holding a pose, they're also paintable, which is something you can use to your advantage.
But one thing you need to bear in mind with any ball joint, but especially the Yellow Sub ones, is that the socket needs to be able to flex, particularly when inserting or removing the ball from the socket. So you can't cut a hole, put a ball socket in, fill the gaps with SGT and then plug your ball strut in - there's a fair chance that you'll either be unable to get it in, or that the pressure will cause your installation to crack. This is less of a problem with polycap sockets but still something to watch out for.
If you look at that picture of my Strike there's a Yellow Sub ball joint there between the torso and the arm: it's gray in the picture 'cause I painted it. In that installation I actually installed the ball into the chest, and attached the socket to the arm, leaving space in the chest for the socket to move around the ball: that way, the back side of the ball socket is the part that's visible - it looks featureless and bland, but it doesn't look like a gray-painted clear ball joint with paint scraping off. Plus I don't have to worry about pressure on the inside of the ball joint causing the chest block to crack apart: but most importantly, this design also features a polycap inside the chest block, so that ball joint can slide into and out of the chest, much like the MG Zaku F2's shoulder - so when I need more posability I can slide it outward, and when I want the arm to look more normal I can slide it back inward. It's a very fiddly system but it's both good-looking and posable.
Another resource I must recommend is
Bawoo's site. Check out his Hi-Zack and Gaz R/L builds. He tends to use
Kotobukiya T-Joint polycaps to build his own hinge joints - these are a good, all-purpose polycap part, they're useful for all sorts of things. For instance, check out his
Gaz R/L elbow joints. They look a bit bland IMO, but they're both simple and functional.
