If there is rust and it's not through the metal, wire wheel it down and use Chassis Saver on it. This will stop further rusting. If it is two seams that meet and need to be sealed, try this, it works great.

Seam Sealer


1959 Chevy Apache Stepside 4x4
2009 Chevy Silverado 2500HD Duramax/Allison LB Crew Cab