Plastic tank? Don't do it. I thought long and hard about pressuring my custom PVC tank (Sch 40 or Class 200) with about 5-10psi of air on a regulator. Did some research and realized that compressed air contains a whole lot more energy than does compressed water. Why? Water is a liquid (Duh!) and does not compress. If you accidentally over pressurize that tank and it lets go, the damage and/or injury could be significant. Not worth the risk. I use gravity feed and mine works fine from the roof rack on the truck, for casual washing and even the occasional shower.