Our airport doesn't have a wash rack or public water supply. I will occasionally use the dry washing products, but the airplane was beyond that. Had bird droppings, dirt, etc. I came up with this contraption to tackle the situation. Thought those of you who are water limited might be interested. It is basically a boat wash pump mounted to a 20 gallon storage container. The wash pump is intended to be mounted on a boat for washing the deck. I adapted it to run on the 12 volt accessory plug in the car. The pump shuts off automatically when there is no water demand. 20 gallons seems to be enough for a basic wash if I take another 5 gallons for the bucket of soap. I need to do a better job of sealing the top, as I made a mess in the car driving to the airport. Wash pump is available at a boat supply store. Prices for the pump start at just under $100. A sump pump might work, but you would need to add the shutoff feature or some form of by-pass.