Did you stretch the weld and the area that turned blue next to the weld and then smooth with a hammer and dolly after welding? The weld shrinks, so the disc is used later after the panel is smooth but high!
If the metal is turning blue with the shrinking disc, you have gone to far. It is only necessary to have the metal steam when it is quenched after using the disc.
It sounds like you might be expecting the disc to do what your hammer and dolly should be doing.
Let me know if this is not clear.