welder has qualification on welding to 9mm thk plate,can he perform welding on 13 mm jointing?

Answer 1

I'm not a welder. I did have a father-in -law who was. From our conversations, I don't see why you can't.
I remember how he used to tell me the importance of filling the weld with no gaps.
If you can do this with 9mm, I don't know why you cannot with 13mm. Maybe you work slower?
There may be much that I don't know.