Great build and great progress, PG!
As far as putty goes, on my MaK builds, there are two kinds of puttying jobs, and I use different putty for each.
For seams, like the seams on the leg or arm halves, on the torso-seams that are like seams on kits of other subjects-then I use a solvent-based putty, as I do on those other kinds of kits. It used to be Squadron white (the old/original formula), but my tube eventually aged and hardened. So I switched to Tamiya's putty. I am also trying Perfect Plastic Putty, but I prefer Tamiya's putty for this job, at this point.
The other puttying job is for replacing the kit's joint covers. The kit parts are OK, and if you want to retain joint mobility, then they're fine. But most MaK builders replace them, most often by using a sculpting putty to cover them and then shape them to taste. I use Aves ("ay-veez") Apoxie Sculpt, but there are other products which are probably as good-Miliput, Green Stuff, among others. Apoxie Sculpt is a 2-part epoxy, with the putty and a hardener, that you mix in equal proportion. I pack that into the joints and then use a toothpick to shape the folds.
I also use Apoxie Sculpt to add weld beads. On the old Nitto kids, weld beads are sometimes molded on, but they're small and sometimes kind of faint. With putty, I can add ones that look better. I'll roll a small, thin "snake" of the putty, lay it down on the model where I want it, and then use a knife blade to make the "beads".
Another tip is to replace the kit's power hoses. They're made of that soft plastic, too, and getting paint to adhere is problematic. There are various techniques here, too. Some guys use fine springs around a fine wire core. In fact, the old Nitto kits provided springs and wire for this purpose. Springs are available from makers of race car kits, like Tamiya, Fujimi, and others.
I like to use wire solder, wrapped with fine wire, to replace the power hoses. Wire solder is soft enough to bend to various shapes, but not so soft that it can be distorted by accident. I use fine beading wire to wrap the wire solder core. As far as painting goes, I find that my usual Tamiya surface primer adheres well enough to the wire, to support the finish colors. I have also used Mr Surfacer to prime the hoses, and I've used thinned white glue, too. That had the effect of filling in the loops of wire a little more, too, which improved the look.
Beside those tips, another I can offer is: throw nothing away, save any odd bits of things, because you can scratchbuild and add details to these kits. That's part of the beauty of Maschinen Krieger. There is "canon", in Kow-sensei's original designs, but there is a lot of room for imagination, too.
I look forward to seeing your updates on this build!
Best regards,
Brad