Bob is right . so much of this comes with experience. I bet most who have been doing this for awhile would probably tell you to spread the whole thing at one time. You dont have to of course, you could do a foot or two section, sand straight, do another section, then when it all feels decent, spread one skim coat over whole shabang to fill any little spots as well as getting each little section you have sanded to blend in good with one another. Of course it will take more time to do it this way. With experience you know about how much hardener to add that it will harden in a reasonable amount of time (0r fully harden at all) that will also give you time to get the filler on and work it nice with the spreader. With experience just by looking at the color of the mixed filler, you can tell about how hot of a mix you have.
How far up from the weld line is going to need filler? For speed you may want to first get all the filler on the weld line area by gobbing it vertically along your weld line about the distance you need fill, then if you have a big enough spreader that is wider then the area you are spreading, go lengthwise along the seam from one end of your weld line to the other with more pressure on the edges so its easier to feather in the filler and less pressure in the middle so it leaves some fill in the area it will most likely be needed. If you get too much of a buildup of filler, take the spreader and get rid of the extra filler on your mudboard, and make another pass one end to the other. I've even used drywall spreaders at some jobs where we had huge areas to fill, The width of one of these should be larger then the area around the weld you need to spread, and with one pass you can smooth the whole works, without working so much. With experience though, a smaller body spreader will be easy to work with also. Hope you kind of understood what I was trying to say. If it isn"t really warped in far from the weld line, well a small body spreader will work well, and also have less to sand. Not seeing exactly what you are doing makes it harder to give advice.