WD40 should prevent rust nicely.

I just overhauled a GH6 the other day. They are really robust hubs; for all the hand wringing I've read on the internet (on this forum, in fact) about demagnetized magnets, I've never encountered one. What I have encountered, however, is stripped axles. The whole axle will be fine, except for 2 mm where the cone locknut sits; that part is stripped, so the cone locknut just spins.

Fortunately the axle is a common thread, 10 mm x 26tpi. Cheap freewheel hubs often have solid axles this size; if you want to go hi-tech, you can even use a QR axle.

I also have a MKII dynohub from the 1930's, which puts out 12v. The design is the same as a GH6, but the proportions are different; the MKII is a lot bigger. To remove the dynamo unit, like the GH6, you unscrew the little nuts from four thin bolts, and the whole dynamo should drop out. Well, mine doesn't drop out, and I haven't figured out any way to get enough purchase on the thing to pull it out with force. I doubt a lot of force will be needed, but I can't apply any, not even a little. Very frustrating. In lieu of a complete overhaul I have dripped oil into it (there's an oil hole in the shell) and massaged grease through the gap by the cone, hoping it will find its way to the bearings... but an overhaul is still needed.