Metal filling on a shaft

Neither. Loctite a sleeve on the shaft. Welding will cause shaft distortion. Since sleeve has to just fill a gap, there are no forces that will cause it to slip. Chill the shaft, heat the bearings and slip them on. Might need to afix the sleeve and grind it to size but Loctite will handle this as well.

why cant those rocket scientists just find a bearing that will fit the undersized shaft, I would do it for them, but saving the rest of the world has got to get completed BEFORE lunch today!

The shaft could be machine welded using a bortech and then ground finish.

This seems to be the obvious best solution in my opinion. It would be a quick and fairly simple fix, and also maintains OEM specs.

If I were tasked with this, I would look at flame spray to build up 1 mm on the dia. Then grind to size. Cheaper than removing a lot of stock, turning a sleeve,welding then machining to size. There is little residual stress from flame spray, and very little distortion. The HAZ of the weld would be of concern for fatique life.

Restoring the dimensions by adding a sleeve is the approach for a long term repair I would go with. I would not recommend weld build-ups on journals, because they increase the stress concentration factor at the edge of the weld, and can lead to initiation of fatigue cracks. Its fairly common to have to repair bearing journals in my industry and most of the time its without removing the roll or piece of equipment. In the past we repaired a wire turning roll journal with a sleeve, the sleeves wall thickness was 0.125" and installed with a shrink fit of .002". The roll weight is roughly 13,500 lb, the journal diameter was just under 6", its about 216" between bearing centers and its turning speed is about 350 rpm, fairly simular to your situation. The details that need to be considered are the fillet radius at the diameter change after the journal is machined, the interference fit and the sleeve material. The fillet radius should be generous and the sleeve material needs to be seamless carbon or low alloy steel (a shrink-fit sleeve has high circumferential stresses). Hope this point of view helps.

spray weld the shaft and then cylindrical grind to the + - tolerences as required.

Your address pls

I'd use a sleeve with press fit agains the fan and the shaft, but, as you mentioned you have only 0.082mm out, your sleeve could have too little wall and/or you would have to machine the shaft too further. If it's not a problem, just go on: Machine, sleeve, loctite, heat sleeve, cool shaft. Everything done.

If you cannot do more machining, go for metal spray, or even some sort of galvanic process like nickel plate or chrome. Think about it.