I get huge incoming files ( up to 6GB) and they are littered with Control and non-ASCII characters. I need to strip them and I made this routine (below). The problem is that it is insanely slow. I would love any thoughts or advice on how I can speed it up.

    public void StripHighBitCharacters(string fn)
    {
        string writeFile = fn + "B";
        using (var reader = new StreamReader(fn))
        using (var writer = new StreamWriter(writeFile))
        {
            while (!reader.EndOfStream)
            {
                string line = reader.ReadLine();
                if (line.Length > 0)
                {
                    writer.WriteLine(BuildClearString(line));
                }
                else
                {
                    writer.WriteLine(line);
                }
            }
        }
        File.Copy(writeFile, fn, true);
        File.Delete(writeFile);
    }
    public string BuildClearString(string line)
    {

        StringBuilder sb = new StringBuilder();

        foreach (char c in line)
        {
            if (c >= 32 && c <= 175)
            {
                sb.Append(c);
            }
        }

        return (sb.ToString());
    }