To add to this thread:
I was working on the inner wing, the part that's inside the cabin above the front parcel shelf. The prior owner had welded a patch in here which had thick weld on the outside and hardly any penetration through to the inside so, I dressed the weld inside and out.
Needless to say, pinholes appeared that I then filled using a very low setting on the welder.
Blowouts. Damn.
I practice the fine art of air-welding again and fix them all, bar one. That one that just keeps getting bigger. So, I stop welding, cut out around the blowout and made a template with new steel. I then set about welding that in.
All went pretty well until - you guessed it - blowout.
I fixed that, then found more pinholes. It's like groundhog day. So I decided to check the state of the section I had cut out.
In the main, it was about 0.8mm thick except for one edge (the edge that blewout), that was only 0.4mm, with traces of rust. Back to the car and, sure enough, the blowout area had rust around it too.
I wonder how many times we actively consider just getting a new bodyshell?
very useful posts