Here is another option I have used in the past under Jeep fenders to prevent corrosion from desert salts. Use the water base "Henry's" roofing patch. It is a asphalt product that is homogenized with water for easy application and cleanup. When dry it leaves only the asphalt and is impervious to everything but petroleum solvents. Very inexpensive, and can be applied in multiple coats for the thickness desired.