Like most others have said, paint it. I like the matte finish. My 322 that I have had for years, never repainted, has a dull matte factory finish. I too have used Testors Dull Coat a lot. If I am weathering anything in chalk, Dull Coat is always applied after I wait a couple of days to see if I like the look. You can always take off the chalk but not after spraying the Dull Coat. Some times after I think I am satisfied with the look and sprayed the Dull Coat I may go back and add more weathering over the Dull Coat then spray yet again. I am a big chalk advocate. I have only renumbered 2 engines using the dry transfer and sprayed the Dull Coat over them with great results.
Kenny