Magnesium, far different than aluminum, builds up stresses from being used as a wheel; it flexes in use and eventually becomes brittle. The stresses cause a micro-pore in the casting to become a crack. TIG-welding, followed by annealing is the only sure way to fix a cracked or pitted mag' wheel, and it takes an expert in TIG to grind out all traces of the crack/pit before welding. Otherwise, since mag' also expands about 2X that of aluminum, the crack will continue to unzip itself in front of the weld bead and you can chase a tiny crack halfway around the wheel. Note the huge casting is safe to work on but the fine grindings from crack removal are extremely flammable!
a TIG-welded wheel also needs to be firmly strapped down with 6 or more c-clamps; my welder used to say 'Campy wheels will crawl off the welding bench if not held down'! A loose wheel can heat-distort into a 'potato chip' as well. For your expert welders' use, Campy street wheels (including the shown Gr-5) are cast of AZ-91 alloy while Campy Gr-4 real-racing wheels are cast of ZE-41alloy. Use the appropriate welding rod.
As a preventative measure on undamaged Campys, I suggest annealing every wheel at home about every 10 years; it's easy and can be done in a kitchen oven. All you need is at least 375 degrees F for 3 or more hours, followed by VERY slow cooling. I take the hot wheel out after 3-4 hrs, wrap it in a wool (not synthetic) blanket, put the still-hot wheel back inside, turn off the oven and leave alone till its stone-cold the next morning. There will be an odor.
This technique comes from aircraft repair shops working on military fighters that also use magnesium wheels, and from Lockheed Space Systems. It removes all residual stresses from age or weld repairs on mag', but causes the protective silver paint to turn tan, so it will then need to be repainted. Do NOT strip the old primer- it is difficult to replace today and is self-healing to a point. Modern water based primers just aren't as good.
All this was detailed in a POCA Newsletter article, Feb 2018, available in the Club Archives for download, including (for Brits) a recommended shop in Bishopthorpe, England that does this work. Good luck- J DeRyke