Welding bananas (or, another thread on distortion)

[Legion]

I'm still very much a beginner, but I can lay down a reasonable bead these days on most things. Now I need to learn about fitting. I remember reading a quote on here once about distortion control/fitting is what separates amateurs from pros (or something similar). Yesterday I needed to make a crude roofrack thing. I have roof bars on my car, but I needed to carry things that wouldn't fit on them (i.e. camping stuff, lots of small things). So I took a piece of 100mm x 100mm x 3.15mm (?) weldmesh (heavily galvanised) and cut it down to 2400mm x 800mm, and I took some pieced of 2000mm long 20mm NB pipe (~26.9mm OD) and some 2000mm long 20mm x 3mm flat bar and made something where the pipe and bar runs down the outside of the mesh, along most of its length centred about the middle. In cross section the sides look like the attached image:

rack.jpg

The mesh is the small diameter circle and I struggled to weld it to the pipe, tacking with 2.6mm Kobe RB-26 rods at a nominal ~68A on a BOC Smootharc 130 with leads set up for DCEN. There's just not enough material to dissipate heat and I suffered a few burn-throughs. In the end I gave up with those and ran a few beads with the 1.6mm Gemini (6012?) rods at nominal ~50A, which worked better.

Because that left a mess, and because I know I can run a clean bead (using the RB-26s) on the pipe/bar join I used that to cover the mess. Lots of tacks and it was all still straight. But when I go to weld, and use a stitching technique of one end, then the middle, then halfway between, etc, etc, with no cooling and no clamping and no tack welding a heavy piece and no clamping of heatsinks I ended up with two big bananas after running welds the whole length of the pipe/bar. This sort of thing happens all the time to me.

I'd burn rod after rod, depositing too much material (probably), running a whole rod then only stopping to get another. So I was welding maybe 150-200mm welds at a time. I knew in advance this was going to happen but I just wanted to do a quick and dirty afternoon job and it doesn't really matter. However, I'd like to know the way to avoid it. I didn't want to sit around for ages between rods. I didn't want to use a bucket of water and a rag and take off my gloves between rods. I didn't want to tack a heavy piece in several spots to the back because it's nicely galvanised pipe and I wanted it to stay clean. I don't want to run beads down the opposite side just to straighten it. I don't have oxy-acetylene to heat it and bend it straight. I can do any of those things if required, but I want to know what the right way to manage this situation is. So what should I have done?

[thesupervisor]

all I can suggest is tack it all up square then do smaller welds say 50mm long but 2at one end then move to opisite corner ect till done
if you have a big table make some temp dog clamps to hold it true but still will needto move around a fair

[KBs PensNmore]

You are putting on FAR to much weld on the mesh, 20-25mm X 100-150mm apart is about as much as I would put on. Putting weld on the intersections of the mesh, is probably the strongest way to go.
Starting in a corner, lightly tacking as you go along (about 12mm every 300mm, just enough to hold it in place) work from the corner along both edges, working your way around until you get to the opposite corner. Then go back and put in the rest of the welds about a metre apart working your way around and go around again and again. This is the only way that I have found after 40 odd years to minimize distortion, even then there could be slight movement.
Kryn