eekpod
It depends what you want it to look like or do, (wadded up toilet paper stuffed in, Bondo utilized sanded smooth and painted, wind and moisture proof).
I got this from AISC Site
Weld access holes should not be filled with weld metal -- doing so could create the very same cracking problems the weld access hole was used to prevent. If the hole is to be filled for appearance reasons in architecturally exposed structural steel (AESS), it is possible to use a body filler such as "Bondo". Refer to Section 10.4.1 of the AISC Code of Standard Practice (a free download from www.aisc.org/code.)
Good Luck
Marshall
10.4.1. The Erector shall use special care in unloading, handling and erecting the
Structural Steel to avoid marking or distorting the Structural Steel. Care shall
also be taken to minimize damage to any shop paint. If temporary braces or
erection clips are used, care shall be taken to avoid the creation of unsightly
surfaces upon removal. Tack welds shall be ground smooth and holes shall be
filled with weld metal or body solder and smoothed by grinding or filing. The
Erector shall plan and execute all operations in such a manner that the close fit
and neat appearance of the structure will not be impaired.