Optimize the string. Format + replace

I have this function. Visual studio profile tagged line with line. The formatting was just as hot and I was spending most of my time.

How can I write this loop more efficiently?

    public string EscapeNoPredicate(string sz)
    {
        var s = new StringBuilder(sz);

        s.Replace(sepStr, sepStr + sepStr);
        foreach (char v in IllegalChars)
        {
            string s2 = string.Format("{0}{1:X2}", seperator, (Int16)v);
            s.Replace(v.ToString(), s2);
        }
        return s.ToString();
    }