If you're passionate about welding you might have an Instagram account and follow some of the great welders out there sharing their work. One who you have to look up to in my opinion is definitely Mike Rubick @six1sixwelding. Here's is a clip I've re-posted on the page sometimes ago :
Mike is working as a tool and die welder, repairing worn out molds or fck ups of engineers and machinist, or just making design change is an cost efficient way. His job consist mostly in building/rebuilding up materials in an additive manner where material is missing before sending it back to the machining shop.
Here’s a good example of how he proceeds to make it happen. First he "draw" contours, basically building walls of welds to define boundaries, then fillet welds them to be sure there is no lack of fusion in the tight spots.
After that he start filling the area, on previous image he's halfway through the first layer.
Here's the result when he's actually filled the whole thing up.
Just like all great craftsmen he's not someone settling for average. He decided then it needed to recap it and then cracked up that stunning finished product.
Notice the use of a thermal insulator to keep the heat up through the process. Molds are not made with mild steel generally and one needs to keep temperature under control to keep material properties intact. The use of appropriate filler metal is crucial too, for a good service life longevity of the mold.
That's some great work, hope you enjoyed the read.