I've used this stuff called EvapoRust before. It's all natural apparently and made from orange peel or something... worked really well for the job i was working on. I used to have a 1985 Honda Prelude with real bad rust around the sunroof... I just hacked it out and treated the rust with that evaporust, welded in a plate and bogged over it. It lasted around 3 years and no rust return (i sold the car and it still wasn't rusty... may be rusty now).

To get rid of mud/clay I take a bucket load of change to the local DIY carwash and blast all that crud off. We have water restrictions where we live atm so we get busted if we get caught washing/degreasing at our house.