I can totally understand and sympathise with your pain. Please do not lose sleep over it. It can be patched up. Other nearby parts of the chassis are clean with no rust,so maybe moist air pocket / wet area was left over while underbody coating was done.That air pocket caused repeated or continuous collection of water,resulting in rusting.
But any good metallurgy worker can repair it in a very good way,with no issues in future. Afterwards the welded area can be painted and all. Since rest of the chassis is alright,you need not worry about future cracks building up on stress areas (provided you do not overload the car or use it very-very roughly).
But any good metallurgy worker can repair it in a very good way,with no issues in future. Afterwards the welded area can be painted and all. Since rest of the chassis is alright,you need not worry about future cracks building up on stress areas (provided you do not overload the car or use it very-very roughly).