http://www.rapidairproducts.com/ This is a product similar to some stuff I sold to county shop. It has a lining on the inside. Steel pipe will start to rust on the inside eventually and then you will end up with rust in your tools. Pvc is cheap and easy and has probably been used 1,000 times. I believe the oil from the compressor may eventually break down pvc. Many people do not realize that there is a proper way to install airlines. If you plan to paint cars I would recommend some good moisture filters. I would also put drains on all your lines that drop down from the main airline. I hate to see people spend thousands on a shop and then cheap out on the important things. Just thought I would through in my 2 cents,,