passing a string between methods

//***************sends string trom txt box to serial
private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{
    if (e.KeyChar == 13)
    {
        string ErrorMessage = Sendtext(textBox1.Text);
    }
}

public string Sendtext(string ToSend)
{
    txt_ammend();   //calls method                               
    try
    {
        foreach (char sb in ToSend)
        {
            serialPort1.WriteLine(sb.ToString());
            Thread.Sleep(950);
            textBox2.Text = sb.ToString();  //this only displays last 
                                            //char of string
        } 
    }
    finally
    {
        serialPort1.Write("x"); 
    }
    return "OK";
}

//**************************stringbulider method
string txt_ammend()            
{
    StringBuilder sb = new StringBuilder();
    foreach (char c in textBox1.Text)
    {
        sb.Append(c);
        textBox3.Text = sb.ToString();   //this shows correctly
                                         //ammended string
        if (!char.IsWhiteSpace(c))       //ie axbxcxdx etc
        {
            sb.Append("x");
            //Thread.Sleep(500);
        }
    }
    return sb.ToString();
}