Many thanks for all the suggestions. I can get 90 grit at Tractor Supply but I was under the impression that 120 was the stuff to use.
Anyway, the M4agery's extension tube is the Guinnea Pig. I had to do a lot of hand machining to get the new buttstock on so I'll try out the process in Graphite Black.
I generally use 80 grit alox before zinc park, then guncote. I have not used ceracote. Watch tractor supply they sometimes stock alox and garnet packaging is similiar.I have had better results and longevity with alox