How to polish welding at 90 degree curves?
That welding would be termed by one of my instructors as “bird droppings”...

It needs to be done properly where the weld joins and fills the 90 gap evenly and is called a fillet weld.

Get them to grind it out and do it properly. A 4” angle grinder with a new disc will get into that.

Have a look at some of the welds shown here: https://www.youtube.com/watch?v=FrS_yafue6M