I just did my garage. I assembled a 10'x14' shed behind the garage where I have put the compressor. Before the slab was poured, I ran 3/4" 'black pipe' from the back wall of the garage, down 3 feet, across 3 feet, then up into the shed. I wrapped it with underground tape and epoxy (just as if it were a natural gas service line). Then, after it puched through the garage wall, I used copper 3/4" line with several union fittings and the rest soldered together. That way, if I need to, I can disassemble it. I plumbed in 2 'dump' valves so I could drain any condensation, and a master valve at the compressor so that the garage could be isolated. The sweated fittings, according to an expert, can handle upwards of 600 psi. I used a combination of teflon and pipe dope on all the threaded fittings.
Tim