My 93 f-150 has a bad case of underbody rust.Whats a good way to get rid of the surface rust and what should I coat it with to prevent further rusting?

 

I had the same prob with my 89. I picked up this green stuff from Advance Auto for 8 bucks and sprayed it all over the underbody. It apparently caused a chemical rx with the oxidized metal that hinders the rust process for furthering itself. Then I allowed it to dry and coated everything rusty with black undercaoting in a spray can for 3.50 a piece. After 6 cans she looks pretty good.