CAUSE
When you use the Borders method on a selection, and there is already a
border around the selection, if you set only the Color or the Weight
property, two of the borders of the entire selection are formatted
differently than the rest of your selection.
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For example, assume that the range B2:C4 on a worksheet has been formatted
with a thick green outline border. If you run the following procedure:
Sub Color()
Dim x As Object
Set x = Range("B2:C4")
' Set color of all cell borders in range to red.
x.Borders.Color = RGB(255, 0, 0)
End Sub
the result is a thick red border around every cell in the selection,
but a thin red border on right and bottom of the selection.
If you run the following procedure:
Sub Weight()
Dim x As Object
Set x = Range("B2:C4")
' Set thickness of all cell borders to thin.
x.Borders.Weight = xlThin
End Sub
the result is a thin automatic color border around every cell, but a green
border on the top and left of the selection.
WORKAROUND
To avoid having unexpected results when you use the Borders method on a
selection of cells on a worksheet, apply both the Weight and Color
property as in the following example:
Set x = Range("B2:C4")
With x.Borders
.Color = RGB(255, 0, 0)
.Weight = xlThick
End With