I just want to give you a quick update. I managed to change the acetylene-supplier (not that easy in Germany). My old bottle was made in 1993 and the manager of my old supplier confirmed that they do not clean out the bottles, just fill up the acetone.
The new bottle I got from a different company was made in 2016...
What else can I say...no more pits in the weld:
20170914_223158.jpg
The small spots you can see were caused by me beeing impatient. Did not clean it well enough. They are really small and are right above the seam (where it was dirty).
If anybody from Germany wants to know, the new bottle is from Globalgas.
Thank you all for the support.
Cheers,
Andy