Rogers, I like your method with the mesh tailgate. I suggest try to get it as close to the rocks as possible so you are not moving them more than a foot. If you can enclose the tailgate or trailer with boards and rock it back and forth a few times--so much the better. Maybe drive down a bumpy road with the trailer. Blow the dirt off, let it dry and blow again. Spray with a powerful detergent--car wash or phosphate or whatever--then power wash to rinse. Coin-op car wash? Probably too many quarters--but they have good pressure, good soap, and possibly hot water. Park the trailer in the hot sun with low humidity to dry the rocks. Walmart in Phoenix comes to mind.