[Pete]

I doubt there is any mechanical bond spray metal that will stand the required torque to the nut.
If you apply any fusion bond metal you are going to draw the heat treat down. This won't hurt anything if done right.
There is a method that works but it requires removing the axle and it requires a welder that can follow instructions. I have done hundreds of these since the 50's so I know it works.

Glass bead the end to clean it completely.
Get some 1/16 Airco 10016D2 arc welding rod and scrub the flux off of several rods with a wire wheel.
Wrap a wet rag around the axle ONLY in the keyway area especially at the upper (big) end. This is where they break most often.
Preheat just the end of the axle to 200 degreesF.
TIG weld the thread area up enough so new threads can be machined. WELD IN SMALL INCREMENTS TO KEEP THE HEAT DOWN. Keep the rag wet. Do not get in a hurry. DO NOT let the taper area turn color.
When done welding, let cool under a blanket.
Machine new thread.
Lap hub to axle with valve lapping compound till you get at least an 90% contact surface.
Clean lapped surface with lacquer thinner or brake cleaner.
Install axle.
Clean lapped surfaces with lacquer thinner or brake cleaner and install hub DRY with new key
and torque new nut to 225 ft. lb. If you can't get the cotter pin hole aligned at 225, PULL MORE till the hole lines up. DO NOT loosen the nut to align the hole.