Add watermark image in c#

I have this little code to add a watermark image in a doc file, only have a little issue, if i convert it one time it works ok, but if i try to convert another file gives me this error:

The RPC server is unavailable. (Exception from HRESULT: 0x800706BA)

in this line:

wordFile = oWord.Documents.Open(row.filename);

This is my code:

Microsoft.Office.Interop.Word.Application oWord = new Microsoft.Office.Interop.Word.Application();
        Document wordFile = new Document();

        //OTHER VARIABLES
        Object oClassType = "Word.Document.8";
        Object oTrue = true;
        Object oFalse = false;
        Object oMissing = System.Reflection.Missing.Value;

private void BtnInserirImagem_Click(object sender, RoutedEventArgs e)
        {
            foreach(MyDocsList row in dataGridList.Items)
            {
                wordFile = oWord.Documents.Open(row.filename);
                addWatermark(wordFile, row.filename);

                //QUITTING THE APPLICATION
                oWord.Quit(ref oMissing, ref oMissing, ref oMissing);
            }
        }

        private void addWatermark(Document doc, string filename)
        {
            object refmissing = System.Reflection.Missing.Value;
            string watermarkPath = Directory.GetCurrentDirectory() + "\\fundo.jpg";

            if (File.Exists(watermarkPath))
            {

                object linkToFile = false;
                object saveWithDocument = true;

                WdHeaderFooterIndex hfIndex = WdHeaderFooterIndex.wdHeaderFooterPrimary;

                HeaderFooter headerFooter;

                for (int i = 1; i < doc.Sections.Count + 1; i++)
                {

                    if (doc.Sections[i].Headers != null)
                    {
                        headerFooter = doc.Sections[i].Headers[hfIndex];
                    }
                    else if (doc.Sections[i].Footers != null)
                    {
                        headerFooter = doc.Sections[i].Footers[hfIndex];
                    }
                    else
                    {
                        headerFooter = null;
                    }

                    if (headerFooter != null)
                    {

                        try
                        {
                            Microsoft.Office.Interop.Word.Shape watermarkShape;
                            watermarkShape = headerFooter.Shapes.AddPicture(watermarkPath, ref linkToFile, ref saveWithDocument, ref refmissing,
                                                                            ref refmissing, ref refmissing, ref refmissing, ref refmissing);

                            watermarkShape.Left = Convert.ToSingle(WdShapePosition.wdShapeLeft);
                            watermarkShape.Top = Convert.ToSingle(WdShapePosition.wdShapeCenter);
                        }
                        catch (System.Runtime.InteropServices.COMException e)
                        {
                            MessageBoxResult result = MessageBox.Show(String.Format("{0} - Error Code = {1}", e.Message, e.ErrorCode));
                            throw e;
                        }
                    }
                }

                oWord.ActiveWindow.View.SeekView = WdSeekView.wdSeekMainDocument;
                oWord.ActiveWindow.ActivePane.View.Type = Microsoft.Office.Interop.Word.WdViewType.wdPrintView;
            }
            else
            {
                MessageBoxResult result = MessageBox.Show("Could not find watermark image");
            }

            //THE LOCATION WHERE THE FILE NEEDS TO BE SAVED
            string outstringFile="";
            string pathfile = System.IO.Path.GetDirectoryName(filename);
            string filenameFile = System.IO.Path.GetFileName(filename);
            outstringFile = pathfile + "\\OG" + filenameFile;

            Object oSaveAsFile = (Object)(outstringFile);

            wordFile.SaveAs(ref oSaveAsFile, ref oMissing, ref oMissing, ref oMissing,
                ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing,
                ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing,
                ref oMissing, ref oMissing);


            //CLOSING THE FILE
            wordFile.Close(ref oFalse, ref oMissing, ref oMissing);


        }

Taken from http://social.msdn.microsoft.com/Forums/en-NZ/vbinterop/thread/2c8689e4-4f8c-42c1-b092-f8ae41b9779c

When you first launch Word programmatically, it connects to Word via an RPC server. When you close the document, this server gets closed without your application knowing about it. The solution is to trap an error and re-initialise your Word object. you will then be able to continue.

Edit:

You initialize your oWord object outside and only once. Then you are calling the .Quit() method on it in your foreach loop in the first run after which it is not available anymore. You will either have to remove the oWord.Quit(); in BtnInserirImagem_Click and live with a constantly initialized word object or you could initialize it when needed and call quit on it afterwards.

    Document wordFile = new Document();

    //OTHER VARIABLES
    Object oClassType = "Word.Document.8";
    Object oTrue = true;
    Object oFalse = false;
    Object oMissing = System.Reflection.Missing.Value;

    private void BtnInserirImagem_Click(object sender, RoutedEventArgs e)
    {
        //Init Word App Object here
        Microsoft.Office.Interop.Word.Application oWord = new Microsoft.Office.Interop.Word.Application();

        foreach (MyDocsList row in dataGridList.Items)
        {
            wordFile = oWord.Documents.Open(row.filename);
            //pass the Word App instance
            addWatermark(wordFile, row.filename, oWord);
        }
        // Quitting oWord outside of the loop
        oWord.Quit(ref oMissing, ref oMissing, ref oMissing);
    }

    //added the Word App instance to the signature
    private void addWatermark(Document doc, object filename, Microsoft.Office.Interop.Word.Application oWord)
    {
        object refmissing = System.Reflection.Missing.Value;
        string watermarkPath = Directory.GetCurrentDirectory() + "\\fundo.jpg";

        if (File.Exists(watermarkPath))
        {

            object linkToFile = false;
            object saveWithDocument = true;

            WdHeaderFooterIndex hfIndex = WdHeaderFooterIndex.wdHeaderFooterPrimary;

            HeaderFooter headerFooter;

            for (int i = 1; i < doc.Sections.Count + 1; i++)
            {

                if (doc.Sections[i].Headers != null)
                {
                    headerFooter = doc.Sections[i].Headers[hfIndex];
                }
                else if (doc.Sections[i].Footers != null)
                {
                    headerFooter = doc.Sections[i].Footers[hfIndex];
                }
                else
                {
                    headerFooter = null;
                }

                if (headerFooter != null)
                {

                    try
                    {
                        Microsoft.Office.Interop.Word.Shape watermarkShape;
                        watermarkShape = headerFooter.Shapes.AddPicture(watermarkPath, ref linkToFile, ref saveWithDocument, ref refmissing,
                                                                        ref refmissing, ref refmissing, ref refmissing, ref refmissing);

                        watermarkShape.Left = Convert.ToSingle(WdShapePosition.wdShapeLeft);
                        watermarkShape.Top = Convert.ToSingle(WdShapePosition.wdShapeCenter);
                    }
                    catch (System.Runtime.InteropServices.COMException e)
                    {
                        MessageBoxResult result = MessageBox.Show(String.Format("{0} - Error Code = {1}", e.Message, e.ErrorCode));
                        throw e;
                    }
                }
            }

            oWord.ActiveWindow.View.SeekView = WdSeekView.wdSeekMainDocument;
            oWord.ActiveWindow.ActivePane.View.Type = Microsoft.Office.Interop.Word.WdViewType.wdPrintView;
        }
        else
        {
            MessageBoxResult result = MessageBox.Show("Could not find watermark image");
        }

        //THE LOCATION WHERE THE FILE NEEDS TO BE SAVED
        string outstringFile = "";
        string pathfile = System.IO.Path.GetDirectoryName(filename);
        string filenameFile = System.IO.Path.GetFileName(filename);
        outstringFile = pathfile + "\\OG" + filenameFile;

        Object oSaveAsFile = (Object)(outstringFile);

        wordFile.SaveAs(ref oSaveAsFile, ref oMissing, ref oMissing, ref oMissing,
            ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing,
            ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing,
            ref oMissing, ref oMissing);

        //CLOSING THE FILE
        wordFile.Close(ref oFalse, ref oMissing, ref oMissing);
    }