Draw vertical bold lines between some columns in datagridview

I want to draw vertical bold red lines between some columns in a datagridview. I created a form and a datagridview, and this is what I want to get:

But this is what I get, notice the two vertical red lines in column header:

Here's my code:

        private void Form1_Load(object sender, EventArgs e)
        {
            this.dataGridView1.Columns.Add("Col_A", "Column A");
            this.dataGridView1.Columns.Add("Col_B", "Column B");
            this.dataGridView1.Columns.Add("Col_C", "Column C");
            this.dataGridView1.Columns.Add("Col_D", "Column D");
            this.dataGridView1.Columns.Add("Col_E", "Column E");
            this.dataGridView1.Columns.Add("Col_F", "Column F");

            //Add rows to datagridview
            for (int i = 0; i < 50; i++)
            {
                this.dataGridView1.Rows.Add("A" + i.ToString(), "B" + i.ToString(), "C" + i.ToString(), "D" + i.ToString(), "E" + i.ToString(), "F" + i.ToString());
            }

            //Set the width of columns
            for (int j = 0; j < this.dataGridView1.ColumnCount; j++)
            {
                this.dataGridView1.Columns[j].Width = 90;
            }

            this.dataGridView1.CellPainting += new DataGridViewCellPaintingEventHandler(dataGridView1_CellPainting);
        }

        //Cell painting event
        void dataGridView1_CellPainting(object sender, DataGridViewCellPaintingEventArgs e)
        {
            //Using red pen to draw border
            using (var redPen = new Pen(Color.Red, 3))
            {
                //Get the x coordination value of the left line
                int left_x = this.dataGridView1.GetCellDisplayRectangle(dataGridView1.Columns["Col_B"].Index, this.dataGridView1.Rows[0].Index, true).X;

                //Get the x coordination value of the right line
                int right_x = this.dataGridView1.GetCellDisplayRectangle(dataGridView1.Columns["Col_D"].Index, this.dataGridView1.Rows[0].Index, true).X+ this.dataGridView1.GetCellDisplayRectangle(dataGridView1.Columns["Col_D"].Index,this.dataGridView1.Rows[0].Index, true).Width;

                //Get the y coordination value of the top of each line
                int top_y = this.dataGridView1.GetCellDisplayRectangle(dataGridView1.Columns[0].Index, this.dataGridView1.Rows[0].Index, true).Y;

                //Get the y coordination value of the bottom of each line
                int bottom_y = this.dataGridView1.GetCellDisplayRectangle(dataGridView1.Columns[0].Index, this.dataGridView1.Rows[dataGridView1.Rows.Count - 1].Index, true).Y
                               + this.dataGridView1.GetCellDisplayRectangle(dataGridView1.Columns[0].Index, this.dataGridView1.Rows[dataGridView1.Rows.Count - 1].Index, true).Height;

                //Draw the lines using red pen and the x, y values above
                e.Graphics.DrawLine(redPen, new Point(left_x, top_y), new Point(left_x, bottom_y));
                e.Graphics.DrawLine(redPen, new Point(right_x, top_y), new Point(right_x, bottom_y));
            }
        }

If I reduce the number of rows to just a few so that all the rows will appear in the form what it first appears, then the red lines appear just as I wanted. But when some rows are hidden, it's screwed up. What am I missing? 

Thanks.