Here’s my take on it. Bags go first.
The bubble is a hole under the surface, rolled flat during the making
of the sheet. Avoid using that piece of metal or return that part of
the sheet for a replacement.
If you pickled the metal after the bubble ‘formed’ then acid got
inside. and wasn’t adequately washed out.
Rolling it put some acid on your rollers (copper blush happens with
acid on steel).
You know what to do for your rollers now…
Auckland NEW ZEALAND