Lessons learned from my 100 ft / 175 psi system>
Initially used RapidAir but the fittings starting leaking badly roughly 3 months after install. (And that was after a "bubble" leak test during installation)
Replaced with 1/2" galvanized, but vol wasn't enough to drive a media blaster I have at the "end of the line".
Now doing a section by section replacement with 3/4" galvanized. Observations:
(1) Tried the Oatey PTFE-based thread sealant (white, non-hardening, setting) -- interim leaks tests showed approx 15% of the fittings leaking. I've used an almost identical product in the past from RectorSeal with similar issues. Moved to Oatey FasTape and Oatey Megatape (both 3.5 mils thick) and "bubble" tests are coming back with no leaks across the board. It's important to wrap the tape in the direction of the threads.
(2)

While these work, they make disconnects/mods difficult later.
(3)

These have been useful as they provide you a means to "break into" a section easier. I did have one fail a leak test --- probably a quality control problem with the manufacturer.
(4) Filters are critical as galvanized "flakes" are possible. I have Parker 40 micron filters in each section along with a Parker 5 micron filter at the port feeding the spray gun.
Warning: multiple internet sites recommend AGAINST galvanized pipe for the aforementioned "flaking" issue. Many are actually run by the alum pipe players trying to push product. Galvanized and or black pipe (rust prone) both work fine provided you insert filters to protect tools, spray guns, etc from contamination.