Well, you can achieve it trough css.
Example
<%@ Page Language="C#" %>
<%@ Register Assembly="Ext.Net" Namespace="Ext.Net" TagPrefix="ext" %>
<script runat="server">
protected void Page_Load(object sender, EventArgs e)
{
if (!X.IsAjaxRequest)
{
Store store = this.GridPanel1.GetStore();
store.DataSource = new object[]
{
new object[] { "test11", "test12", "test13" },
new object[] { "test12", "test22", "test23" },
new object[] { "test13", "test32", "test33" }
};
store.DataBind();
}
}
</script>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title>Ext.Net Example</title>
<style type="text/css">
.my-grid .x-grid3-header-offset {
padding-left: 0px;
}
.my-grid .x-grid3-row td {
padding-left: 0px;
padding-right: 0px;
border-top: 1px solid;
}
.my-grid .x-grid3-row-last td {
border-bottom: 1px solid;
}
.my-grid .x-grid3-row {
border: 0px;
}
.my-grid .x-grid3-hd-row td.x-grid3-cell-last {
border-right-width: 1px;
border-right-style: solid;
}
.my-grid .x-grid3-hd-row td {
border-left-width: 1px;
border-left-style: solid;
border-color: Black;
border-right-width: 0px;
}
.my-grid .x-grid3-col.x-grid3-cell {
border-left-width: 1px;
border-left-style: solid;
}
.my-grid .x-grid3-col.x-grid3-cell-last {
border-right-width: 1px;
border-right-style: solid;
}
</style>
</head>
<body>
<form runat="server">
<ext:ResourceManager runat="server" />
<ext:GridPanel
ID="GridPanel1"
runat="server"
AutoHeight="true"
Cls="my-grid">
<Store>
<ext:Store runat="server">
<Reader>
<ext:ArrayReader>
<Fields>
<ext:RecordField Name="test1" />
<ext:RecordField Name="test2" />
<ext:RecordField Name="test3" />
</Fields>
</ext:ArrayReader>
</Reader>
</ext:Store>
</Store>
<ColumnModel runat="server">
<Columns>
<ext:Column Header="Test1" DataIndex="test1" />
<ext:Column Header="Test2" DataIndex="test2" />
<ext:Column Header="Test3" DataIndex="test3" />
</Columns>
</ColumnModel>
</ext:GridPanel>
</form>
</body>
</html>