New CNC Mill on the horizon

[Skysurfer]

I'm a Boilermaker, and run into this all the time. The bow is caused by the shrinkage of the steel after it's cooled from the cherry red of the weld. If it's easier for you, if you heat the opposite side to the weld by roughly the same amount of heat that's gone into the weld (you could weld opposite, then grind flush when finished), it will shrink by the same amount (i.e. straighten). I normally put a torch to the opposite side, and eyeball down it's length until it's bob on.
Always weld opposite the last weld to keep distortion to minimum, but it'll always creep in...
You should put a couple of decent tacks on one seam, then weld the opposite. Then go back & weld over the tacks.
Might save you effort and/or time...Certainly save epoxy...
Really love that gantry, presumably ally? Would love some more details on it. Which supplier did you go to for your rails?
Looking good...