Welded stainless pipe can be restored to full passivity by proper heat treating. This usually involves heating austenitic SS to about 2000F and water quenching. Ferritic stainless is heated to about 1650F and quenched. HOWEVER, if the SS is an "L" grade (low carbon, ie, under about...
You might try a slower heating rate, or stress relieving it prior to painting (long time, low temp). Or just clamp it onto a heavy metal section during heating. Just remember to heat and cool slowly.
;You don't say how big the piece is. If it is a forging or large rolled section, decarburization could be the problem. Annealing at high temperature for a long time can do it too. Try grinding deeper and take another test. There are minimum stock removal tolerances on Hot Rolled Bars for this...