Let your welds cool on their own, do not force cooling with air or damp rag, doing so will in fact make the distortions worse and the welds more brittle and prone to cracking than if you let it cool on it's own. Also try a heat sink on the backside of the weld to draw excess heat out. Use Aluminum or Copper as a heat sink, weld won''t stick to it and will minimize warpage, especially on thse flat panels. Regardless of what you do, distortion is imminent, how you deal with it affects your finish. Also turn up the heat a bit more, it would be nice to see better fusion on the backside of those welds, that is an ideal spot to re-introduce rust. Rather than tack beside tack, run a 1/2" to 1" continuous bead to make sure there are no voids. If you do decide to run a series of tacks, cover 1/3 to 1/2 half of the previous tack to prevent those rust causing voids around the tacks.Esab Easy-Grind wire 0.23" is a good choice, not quite as brittle as standard wire plus it grinds easier and distorts less. Try hammer and dolly method (You Tube) to flatten those welds a little meaning less grinding heat build-up and distortion. Warpage is a fact, you're going to have to deal with it regardless, so just keep practicing until your technique produces the least amount of distortion and once you finish welding, your next job will be to straighten out those welds, Every weld is better than the last...practice, practice, practice and keep up the good work, the more you prepare, the better.......there's my 2 pennies worth.