The two most commonly used polyamide grades by far are PA6 and PA66. The remaining members of the polyamide family are primarily used in applications that require unusually high service temperatures or tolerance of moist conditions. Bio-based and transparent (amorphous) PA grades are also available. In machine design purposes PA is often reinforced with glass or - in case of highly demanding applications - with carbon-fiber. Reinforcement changes the characteristics of the material and its applications considerably and therefore I’ll write a separate article on them later. This article focuses on unfilled PA6 and PA66.
Be careful of used tanks as they are being discarded for a reason, out of date for safety may be the biggest of them. Tanks can and do rust from the inside and around the welded areas. Where I work we sell bottles and propane and we have had safety bulletins about a tank blowing because of a weak weld. Luckily no one was killed. Test the tank by tapping it with a non-sparking hammer around the bottom and near the welds while listening to the difference in sound. You may also want to flood the tank with water to rid the tank of the smell before using it for an air tank. After which you may also want to add some alcohol to the tank to get rid of the water then let it air dry in the sun to remove the alcohol.