Proper and elegant way of Building a XML Document
I ended up changing my approach and went with a serialize method:

public static class SerializationHelper
{
    internal static string SerializeObject&lt;T&gt;(this T toSerialize)
    {
        var xmlSerializer = new XmlSerializer(toSerialize.GetType());
        var textWriter = new StringWriter();
        xmlSerializer.Serialize(textWriter, toSerialize);
        return textWriter.ToString();
    }
}
[Serializable]
public class SFDocument
{
    public int TableId { get; set; }
    public int PageSize { get; set; }
    public int PageIndex { get; set; }
    public string MultiCompany { get; set; }
    public int Language { get; set; }

        [XmlArrayItem("Field")]
        public List&lt;string&gt; Fields { get; set; }

        public List&lt;TableFilter&gt; TableFilters { get; set; }

        [XmlArrayItem("Field")]
        public List&lt;string&gt; Key { get; set; }
        public SFDocument(): this(5415, 1, 1, "No", 1033){}

        public SFDocument(int tableId, int pageSize, int pageIndex, string multiCompany, int language)
        {
            TableId = tableId;
            PageSize = pageSize;
            PageIndex = pageIndex;
            MultiCompany = multiCompany;
            Language = language;

            Fields = new List&lt;string&gt;();
            Key = new List&lt;string&gt;();
            TableFilters  = new List&lt;TableFilter&gt;();
        }
}   

[Serializable]
public class TableFilter
{
    public string Field { get; set; }
    public string Filter { get; set; }

    public TableFilter() {}

    public TableFilter(string field, string filter)
    {
        Field = field;
        Filter = filter;
    }
}

void Main()
{
    var sfd = new SFDocument();
    sfd.Fields.Add("Field1");
    sfd.Fields.Add("Field2");
    sfd.Fields.Add("Field3");
    sfd.Fields.Add("Field4");

    sfd.Key.Add("Key1");
    sfd.Key.Add("Key2");
    sfd.Key.Add("Key3");

    sfd.TableFilters.Add(new TableFilter("field1", "filter1"));
    sfd.TableFilters.Add(new TableFilter("field2", "filter2"));
    sfd.TableFilters.Add(new TableFilter("field3", "filter3"));
    sfd.SerializeObject().Dump();
}