What is the "three coat polyurethane system"?

"Polyurethane" refers to the material type and the three coats are as follows: two coats of a rust sealer and one coat of a topcoat, like AG111 or Chassis Black Finish. Two coats of rust sealer and one coat of a topcoat create a barrier to stop rust and prevent it from coming through. You must ALWAYS use two coats of rust sealer, no matter what kind of surface you're working with. If you don't plan to topcoat, use three coats of rust sealer. Anything less will be ineffective.