Rust on the half shaft ends
 

My car doesn't have a differential so the half shafts are just laying there. One end is covered up and seems to be fine, but the other is exposed and has rusted. Could I just wire-brush it clean, or would this be bad for the diff? If a wire brush is out, are there any methods that would be acceptable, like bead blasting?

How badly has it rusted? Try wiping it down with some oil and see if the surface has become pitted.

Like, severely rusted. I would say it's definitely pitted. At the very least it's a solid layer of surface rust.

If the portion of the shaft-end that inserts and mates with the differential side seal is that pitted, it's junk.

Any time I store axles I oil the ends and put a glove or plastic bag over them. If it is very light you may be abler to get it out with some 1000 grit sandpaper. Then step down to 2000 grit. If it is pitted it will most likely tear the axle seal if you try to use it and leak.

The other one had a cotton work glove taped over it and seems to be OK.

I'll take a mild wire wheel to it and see if it's still pitted. I don't think hand-sanding it with 1000 grit would do anything. How tight are the tolerances on these things? So long as the splines fit and it doesn't tear the oil seal, shouldn't it be OK?

I've used scotchbrite and kerosene on the stub of the tripod joint in the past - a bit of surface rust in that case and cleaned up surprisingly easily. Not too sure I'd take to it with a bench grinder or angle grinder with a wire wheel, but it's some alloy of fair quality, so shouldn't be a big issue. I'd expect the area the seal rides on, if pitted, makes it a throwaway.

Having actually tried inspecting it again, it seems like might not be that bad. Plus, I don't have a 26 mm (??) socket to take the shaft out anyway... And I have some kerosene on hand so I'll try that.