Even if this idea wasn't extremely dangerous, WHICH IT IS, the tanks wouldn't work because they wouldn't supply a steady volume of air. The pressure would be high at first, and as the tank got low the pressure would drop, changing what was happening at the paint gun. You would be trying to paint, trying to keep the gun adjusted, and have to switch to a fresh tank in the middle of a coat. It's just not done this way, period. You need to rent or buy a compressor, get a book or video on painting cars, and ask a lot more questions here. As far as contaminents in the paint, well welcome to the painter's world, even the best of us get them under ideal conditions. Learn to paint first, and trust me.......contaminents will be one of your last worries!