Not worried about uv as rats will chew through it first so will be replaced often and it is cheap so i can. Going to use 20mm stuff for now and 25 later on .You need to make sure its rated for uv exposure, it's fine with air in it but I've seen thw stuff that's meant to be buried crack after being in the sun for a few years.
could you cover it in pipe insulation would that help ?
Water Hammer is not generally a problem with compressed air systems because air is compressible
Kinetic energy is Ek = 1/2 mv2The reason I suggested he think about it is if he has 8 bar of air at full flow through the line (maybe he is blasting) and then closes the valve, will the pressure spike come uncomfortably close to the 12 bar rating of the pipe?