Sorry but I'm not following the logic of your code here.
If Sheets("Trim".Visible Then
With Range("D13")
If .Value = "HP-1" then
.Value = "16 GA"
Else
.Value = "26 GA"
End If
End If
The code in this project has been cobbled together for years and I just maintain it...and it has worked this way for years but let me try it this way...