Underground black steel gas line to grill,light and firepit

New member who has read previous threads regarding underground gas lines, but my situation is different. First I realize PE pipe would be the best but I cannot afford the cost of all the fittings I need. Second I understand the fuel gas code does not allow threaded fittings underground but I live in the boonies with a gas well on my property and I will not have to pass any inspections. I want to do the best corrosion protection I can afford. From what I have read on other posts there is a tape that can be applied with a primer, what is this product name and where is it available. Old timers in the area have used "tar" to coat pipe and they do insist that is all I need. Another old post mentioned a heat shrink product, could this be the best for the tee and ell sections? I was also considering backfilling the gas line with limestone rock dust as what I had to dig out for my trenches was very rocky. Could the limestone dust increase corrosion on the steel pipe? Finally I should mention I will have a tee plumbed in an accessible place for a test gauge, isolated from the interior gas lines, to enable periodic pressure testing of the buried lines.

We use threaded connections underground all the time. How else could you install small size gas lines. There are no "fittings" with PE pipe other than the risers at each end that you connect the rest of the piping to the PE. IF yo insist on using steel pipe, use galvanized pipe. Wrap it with overlapping layers of 20 mil pipe wrap, after priming the pipe. IF any sections ot the tape are damaged by wrenches tightening the pipe, recover those areas.