Obviously, Chapter 4 doesn't cover every possible welding repair and
the field is left to interpret the Circular, making up a justification
one hopes will fly with the FSDO (when the 337 is submitted). For
example, the joining of two dissimilar sizes of tubing using a
fishmouth or scarf joint (as in stepping down the size of longerons)
can be the justification for how one might repair the end of a tube
that is badly rusted.

Does the IA course cover interpreting 43.13 or is that a matter of
working with the FSDO? - Mike