I second the motion... Walter Birdsong's Black-T & Green -T are the way to go, if for no other reason than that his application process respects and preserves the integrity of the hard anodized finish of an aluminum AR receiver.
He does not bead blast the anodizing into oblivion prior to refinishing, and that is the reason why his finishes are the only ones recommended & approved of by SIG Sauer for the refinishing of their firearms (direct from Walter's mouth, BTW).
If you check the Shootiniron web site, I believe that Mac states that his refinishing process removes the hard anodizing from aluminum, but that he replaces it with some unnamed "chemical coating", that is by his own admission, not as hard or durable as an anodized finish.
I am not trying to knock Mac's refinishing process (I'm sure it is perfectly fine for steel), but when it comes to aluminum; there is only one game in town...
Walter Birdsong & Associates
1435 Monterey Rd.
Florence, MS 39073-9748
Tel.# (601) 939-7448
And yes, I speak as a past and future customer!