I did this a few months back but never got around to posting the thread. I preferred the look of Siege Ultra Magus's cab bot to the Galaxy Prime version, that small amount of waist taper makes all the difference. Galaxy Prime looks like a cube on legs. I'd seen a few people do this with paint, but I'm not good at painting so I figured I'd give dying a try and the result came out surprisingly well.



All blue parts are donated from a Galaxy Prime, aside from the bumper from the original Magnus. Luckily both figures use the exact same shade of blue plastic. I thought about painting the bumper silver for a more traditional Prime look, but again I'm bad with paint and didn't want to risk ruining it. I did go over the smokestacks with a silver paint Sharpie though, not very well in these pictures but I did another pass later.

I also made one other mod because I never liked that large gap behind Ultra Magnus' front wheels. I cut off the flat panel and wheel from one of Galaxy Prime's front wheel pieces, and the resulting wedge was the perfect size and shape to be superglued behind the wheel. Thanks to Allspark's gallery for an example picture, since I no longer have a complete version of this piece:

This does mean Prime's legs can't move backward from the hip very far anymore, but that's not needed for most poses so I think it's an acceptable sacrifice.

One remaining problem I had with the vehicle mode was Prime's very visible fists, so I hacked up the various grey bits on the Galaxy Prime figure to make a mini trailer/weapons platform thing.



This clips on the "spine" of the cab and can be left on during transformation, since I have a hatred for partsforming, but the end result is pretty ridiculous looking. Ideally I'd like make this fold down against his back, but I don't have the skill to pull that off.

Process:

Disassembly:
For the most part disassembly of the Ultra Magnus cab was pretty easy. There are two pins that have to be removed, one at the front of the cab that allows it to fold open and one that runs through the top of the cab to allow the head to rotate. Both came out fairly easily with a pin punch, they're very long pins but once I got the textured tip out I was able to pull the whole thing out with no more resistance. The only really tricky part was removing the blue windshield/grill piece. Looking inside the cab there are four pin-like protrusions near the top holding it in (in addition to the bottom pin, which was already removed by this point), I tried to just tap these out with the pin punch but in retrospect I think they're glued in. I was able to get it out this way but only with those pins or the plastic around them breaking. Fortunately this isn't visible from the outside aside from one corner of the windshield, were I to do this again I think I'd just dremel the plastic out around the pins. Thanks to the loaner bits from Galaxy Prime I only needed extract the front and back of the cab and the shoulders and forearms for dyeing.

Dyeing:
The guide I found online recommend Rit DyeMore, which is a somewhat new product designed specifically for synthetic materials, and the "Racing Red" color turned out to be a perfect Optimus red. Though it is a noticeably different shade than the Galaxy Prime. The guide also said to use two cups of water to 5% acetone, but this amount did absolutely nothing. I bought a 6oz bottle of 100% acetone nail polish removal, and ended up mixing a bit under half of that to the two cups of water before the plastic started noticeably taking the dye. I bought a strainer and a cheap pot (because obviously don't put acetone in anything you're going to eat out of later), heated the water/acetone mixture to a point where it was too hot to touch but nowhere near boiling, added the dye until I got to a shade I liked, then just kind of jiggled the strainer/rolled pieces with some tongs to prevent them from sitting in one position until they got to the color I liked.

Issues:
The first issue came up during the dyeing process. The "spine" on the back of the cab that the bumper slides up during the transformation is evidently made from a different kind of plastic than everything else, and starting taking the dye much quicker than all the other parts. I think this part could be separated with one more pin removal so it could be taken out of the dye earlier, but by this point I was too far in so that piece ended up a much darker red, almost burgundy.

The main issue with this figure became evident when I was trying to reassemble everything, I'm not sure if it was the heat, the acetone, or both, but the tolerances of the figure all became slightly off. I wasn't able to get the pin at the top of the cab in all the way, and I wasn't able to get the pin at the bottom of the cab back in at all, as the holes in the cab and the windshield part no longer lined up right. Fortunately I had spare pins from cutting up the Galaxy Prime wheel bits, and was able to put one in from each side to kind of hold the windshield in, though they still don't fit in all the way. Also the top pin stuck out enough to damage the shoulder piece that folds into it. You can also see the windshield stress mark that came from hammering it out in the same region.

Because of this change in tolerance the cab figure is no longer able to combine with either trailer. I put one of the Ultra Magnus forearm pieces on one of his arms and was barely able to get it off again as the peg got completely jammed in the hole, so I didn't risk trying any others. And it turns out the Magnus cab can't wear the Galaxy Prime head/chest piece at all, as the top of the cab sticks out too far forward. I'm sure you could put certain pieces of each armor on, but I didn't see a point in trying too much.

The trailer bit also doesn't look great from some angles, as the pieces were never meant to fit together, and there's also some of that white residue that superglue creates, although most of it was able to be cleaned off with a toothpick.


Overall though for my first attempt at dyeing I'm very satisfied with how this whole figure turned out.

 

Excellent. I thought of doing this too, cuz I loved the siege magnus Voyager bot. He has such a beefy stock to him that works well for a prime. It didn’t transfer over all that well for Galaxy Prime. I’m surprised hasbro didn’t release an Optimus Prime version of magnus.